Space storms
1
are the dominant contributor to space weather. During storms, rearrangement of the
solar wind’s and Earth’s magnetic field lines at the dayside enhances global plasma
circulation in the magnetosphere
2,3
. As this circulation proceeds, energy is dissipated into heat in the ionosphere and
near-Earth space. Because Earth’s dayside magnetic flux is eroded during this process,
magnetotail reconnection must occur to replenish it. But whether dissipation is powered
by magnetotail (nightside) reconnection, as in storms’ weaker but more commonplace
relatives, substorms
4,5
, or by enhanced global plasma circulation driven by dayside reconnection is unknown.
Here we show that magnetotail reconnection near geosynchronous orbit powered an intense
storm. Near-Earth reconnection at geocentric distances ~6.6-10 Earth radii – likely
driven by the enhanced solar wind dynamic pressure and southward magnetic field –
is observed from multi-satellite data. In this region, magnetic reconnection was expected
to be suppressed by Earth’s strong dipole field. Revealing the physical processes
that power storms and the solar wind conditions responsible for them opens a new window
into our understanding of space storms. It encourages future exploration of the storm-time
equatorial near-Earth magnetotail to refine storm driver models and accelerate progress
toward space weather prediction.
The solar wind flow imparts energy to Earth’s magnetosphere that is dissipated as
heat during substorms and storms. Substorms
4
are commonplace (several per day) cycles of solar wind energy capture and dissipation
lasting 1 to 3 hours, occurring under moderate or active solar wind conditions. Initially
stored as magnetic energy in Earth’s nightside magnetosphere, the magnetotail, this
energy is abruptly released as particle energy by magnetotail reconnection, 20 to
30 Earth radii (RE) downtail
5
. Substorms, which produce continent-scale, bright auroras, do not have harmful space
weather effects because they do not substantially affect the ring current (westward-drifting,
tens of keV-energy ions, Fig. 1A) or the radiation belts (eastward-drifting relativistic,
“killer” electrons, Fig. 1A), both of which encircle Earth near geostationary orbit
(at geocentric distances R~6.6RE). Storms are infrequent (about one per month), last
several hours to days, occur under active or very active solar wind conditions (peak
or declining phase of the solar cycle), produce global-scale, most brilliant auroras
and have severe space weather effects
2
, as they are associated with ion energization in the ring current
1
and electron acceleration in the radiation belts
6
. Magnetotail (nightside) reconnection also occurs during storms, but whether it can
power them or is a mere byproduct of the global flux circulation driven by dayside
reconnection is unclear: from 20-30 RE downtail, where nightside reconnection typically
occurs, its outflows cannot reach inside geostationary orbit to energize the storm-time
ring current and radiation belts
7
. Moreover, at such large downtail distances, the magnetic energy per particle in
the lobes (Fig. 1B), Em =miVAL
2 =Blobe
2/μ0Ni, available for local particle heating (expected
8
to be: ΔTi ~13%Em ~3keV) is lower than typical storm-time ring current particle energies
(Note: VAL: inflow Alfvén speed; Blobe: lobe magnetic field ~20nT; Ni: ion density,
~0.1/cm3; and mi: ion mass). If reconnection were to occur much closer to Earth (near
geostationary orbit) it could be more geoeffective and powerful, but it is thought
to be impossible there
9
due to the stabilizing effect of Earth’s strong dipole. Indeed, although on a few
occasions reconnection has been suggested to take place at R<15RE,
10-11
it has been neither incontrovertibly identified nor placed energetically in the context
of space storms. Thus, despite significant advances over the last ten years in our
understanding of energy conversion during their weaker relatives, substorms, whether
storms are powered by nightside reconnection or by the global circulation driven by
dayside reconnection has been an open question since the dawn of the space age. Here
we report the first comprehensive in-situ observations of magnetotail reconnection
directly powering a storm from as close to Earth as ~8RE.
From 03:00 to 06:00UT on 20 December 2015, during the main phase of an intense storm,
the THEMIS P3, P4, P5, and GOES G13 satellites (Methods M1) were near the magnetic
equator (Fig. 1A-B), well-situated to observe magnetotail reconnection. The solar
wind’s large southward magnetic field, Bz,gsm<0 (Fig. 2A), and dynamic pressure, Pdyn
(Fig. 2B), imparted sufficient energy to the magnetosphere to intensify the ring current
and ionospheric heating (attested by ground-based indices in Extended Data Fig. 1A,
1B) to values typical during such storms
1,3-4
.
Immediately following a dynamic pressure increase during the above time period, ground
magnetic pulsations at ~04:46UT (Fig. 2B) signified intensification of magnetotail
reconnection
12
. Around that time, all the abovementioned satellites observed a Bx-dominated (tail-like)
field (Fig. 2E, 2F, 2I), a prerequisite for reconnection. In particular, because P5
and P3 or P4 observed Bx with opposite signs from 04:00 UT to 05:00 UT (Fig 2C), they
bracketed the neutral sheet (where Bx = 0) and were close to it most of the time (as
∣Bx∣<Blobe). Thus, they were well positioned to observe the current sheet and its
evolution, allowing us to compute (M6) its average density, Jy, (Fig. 2D). Peaking
at ~80nA/m2, Jy became at least 10 times greater than during typical substorm pre-onset
times
13
. The current sheet’s earthward-most extent is thus expected to be unusually close
to Earth
14
, explaining why the field at G13’s geostationary location was also tail-like in both
observations and empirical models
15,16
(Fig. 1A-B). All these conditions favor near-Earth reconnection.
At ~04:46:40UT, G13’s Bz began to increase (Fig. 2E) toward Earth’s local dipole value
(Bz,dipole~+111nT) as fluxes of high-energy (>100keV) particles intensified (Extended
Data Fig. 1G-H). Simultaneously, P5 started to observe a bipolar Bz (Fig. 2F), southward-then-northward
(negative-then-positive), correlated with fast, tailward-then-earthward Vx (negative-then-positive,
Fig. 2G) and heating (Extended Data Fig. 1J). (Note that THEMIS plasma data were corrected
for the presence of O+ , in M3, and energetic particles in M4.) Similar Bz and Vx
signatures were observed at P4 (Fig. 2I-J), south of the neutral sheet, though the
tailward flows at P4 were slower than at P5, likely because P4 was farther away from
the neutral sheet than P5 at the time (Fig. 1C). We interpret these observations (Fig.
1B-C) as a reconnection region appearing between G13 and P4 (X=−6.1 to −9.5RE) at
~04:46:40UT and retreating tailward of P5 and P4 approximately 2min later, at ~04:48:30UT
(marked as “X-line” in Fig. 2H-I). The nearly simultaneous initiation of opposing
Bz excursions at G13 and P4 at ~04:46:40UT suggests that reconnection started midway
between them, at X~ −8RE. The estimated tailward retreat speed ((−9.5)-(−8))RE/2min~
−75km/s) is similar to that in previous (substorm-time) reconnection observations
farther from Earth
17
.
During the fast flow period (Fig. 2G) when reconnection was locally active (Δt = 04:45-04:57UT,
horizontal bar above Fig. 2A, C, E) the reconnection electric field ERX=Ey (Fig. 1C)
at P5 was intermittent but consistently positive, comprising intense, ~1min-duration
pulses (Fig. 2H: Ey,peak~100mV/m ~0.1VALBlobe, for Blobe~120nT, and Ni~0.1/cc, the
measured lobe density). These bespeak of rapidly recurring, fast, impulsive reconnection.
Multiple Bz<0 excursions at P5 (Fig. 2F) after the passage of the first X-line were
embedded within persistently earthward flows (Vx>0, Fig. 2G). These could be secondary
(weaker) X-lines swept by the outflow of the first (primary) X-line. We focus our
attention on the 04:46:30 to 04:50:00 UT interval (vertical solid lines in Fig. 2E-J),
which encompasses the primary X-line.
We next establish that this nearly geostationary reconnection event has the hallmarks
18
of an active reconnection region: inflows; outflows threaded by Bz of the correct
polarity; and a Hall system of electric fields, EHS, and currents, JHS, arising from
separation of ions and electrons in the ion diffusion region and leading to a quadrupolar
out-of-plane magnetic field, By (Fig. 1C). As the X-line retreats tailward, the satellites
move earthward in the frame of the X-line (dashed lines in Fig. 1C). When cast in
Bx-Bz space (Fig. 3), their observations can be compared to expectations from reconnection
18
, from either side of the X-line (Bz=0) and/or the neutral sheet (Bx=0), as measured
in sequence by individual satellites, or even simultaneously by multiple satellites.
We find that on both sides of the neutral sheet, the reconnection outflows (Vx) are
directed away from the X-line and the inflows (Vz) are directed towards it (Fig. 3A,
3C, per Fig. 2G, 2J), as also corroborated by ion distribution functions (M5). The
reconnection electric field (Ey, Fig. 3B; Extended Data Fig. 2B, 2D) is strongest
and consistently positive across the entire reconnection region when ∣Bz∣ is large.
The Hall electric field (E⊥,HS~Ez, Fig. 3D; Extended Data Fig. 2F), even more intense
than Ey, points towards the neutral sheet from both sides of it. The parallel electric
field (E∥, Fig. 3F; Extended Data Fig. 2H) observed clearly only at P5 (closest to
the neutral sheet), reverses sign across the X-line. The magnitude and direction of
E⊥,HS and E∥,HS in these observations agree with simulations
19
. A quadrupolar out-of-plane magnetic field, By, at P3, P4, P5 and G13 (Fig. 3E; Extended
Data Fig. 2E) is also consistent with expectations
18
(By,HS, blue arrowheads/tails in Fig. 1C). The associated in-plane currents, JHS,
approximated as Jx~(1/μ0)∂By/∂z from P3, P4, and P5 measurements (M7), flow away from
the X-line at locations far from the neutral sheet (∣Bx∣>>0) and toward the X- line
near it (Fig. 3G, consistent with black open arrows in Fig. 1C). The earthward field-aligned
current is tens of nA/m2. If a fraction of it were to close through the ionosphere
(with a mapping factor of >1000), it could provide several μA/m2, consistent with
bright aurorae
19
.
Comparing the solar wind flux input to the magnetosphere (M8) based on solar wind
measurements, ΔtΦin ~0.2GWb, to the flux transport measured at THEMIS, Δt∫Eydt~0.053GWb/RE,
provides an estimate of the magnetotail reconnection region’s effective width, ΔY~
ΔtΦin/Δt∫Eydt~ 4RE, which is considerably smaller than the magnetotail width (~40RE).
Similarly, comparing the magnetospheric energy dissipation, ΔtUmd~ 0.82PJ, estimated
from magnetospheric activity indices (M8), to the time-integrated Poynting flux into
the magnetotail current sheet as observed at P5, Δt∫Szdt ~0.0228PJ/RE
2, provides an estimate of the effective reconnection area, ΔX·ΔY~ ΔtUmd/Δt∫Szdt~
36RE
2~ 9·4RE
2 (the rectangle in Fig. 1A). The flows observed at THEMIS lasted only ~10min, even
though the storm continued for hours. Multiple activations of tail reconnection at
different azimuthal locations may have provided the aggregate energy conversion over
the entire storm’s lifetime. The width, duration, intermittency, and flux/energy transport
efficiency of storm-time reconnection outflows are similar to those of bursty bulk
flows during substorms
21
. However, fast storm-time reconnection operating near geostationary altitude (where
Blobe~120nT, six times that at 20-30RE) can be ~36 times more effective in energy
conversion than during substorms (since Em∝Blobe
2) and have unimpeded access to cis-geostationary altitudes to efficiently power the
storm-time ring current and radiation belts.
What led to reconnection so close to Earth? The storm-time magnetic model TS0415,
with input from the observed solar wind Pdyn and Bz,GSM (M9), exhibits intense, near-Earth
(X~ −8RE) current sheet thinning and an equatorial Bz minimum (Fig. 1B; Extended Data
Fig. 3). Consistent with our pre-reconnection observations at G13 and THEMIS (Fig.
2E, 2F, 2I; Extended Data Fig. 4), these conditions are conducive to magnetic reconnection.
Since such intense solar wind driving is common during storms, it is likely that near-geostationary
storm-time reconnection is also common, though elusive due to the extreme thinness
of the current sheet (Fig 1C), the rarity of storms, and the scarcity of observations
in this region. Studying these findings statistically and exploring the nature of
energy conversion by retargeting current satellites or with future missions would
be important for advancing our understanding of reconnection in many astrophysical
and laboratory settings as well as for space weather modeling.
Methods
M1. Satellites and instrumentation
The ARTEMIS
23
(P1, P2) high lunar orbiters located in the solar wind at the time of interest and
the THEMIS
24
(P3, P4, P5) high Earth orbiters are identical satellites, also referred-to by their
letter identifiers TH-B, -C, -D, -E, -A for P1-5, respectively. Data were accessed
and processed using the Themis Data Analysis Software (TDAS), a plug-in of the Space
Physics Data Analysis System
22
(SPEDAS) V3.0. Onboard Fluxgate Magnetometer
25
(FGM) and ground-processed Electric Field Instrument
26
(EFI) spin-fits (spin-period, Tspin, is 3-4 s depending on satellite) were used. For
EFI ground processing, Fast Survey
27
voltages (at 8 samples/s) from the long sphere wires were utilized. Spurious spikes
in these voltages due to shadowing by the satellite body were removed, then the voltages
were spin-fit to produce the two spin plane E-field components. Spin-averaging of
spin-axis voltages resulted in the third E-field component. Coordinates are discussed
in the next section (M2). Ions and electrons between ~5 eV and ~30 keV were measured
by the Electrostatic Analyzer
28-29
(ESA); those between ~35keV and ~1MeV were measured by the Solid State Telescope
24
(SST). Neither instrument provides ion mass discrimination. Standard calibration,
background subtraction (photoelectrons, secondary electrons, internal scattering,
and penetration), and satellite potential correction were implemented for the ESA
29
. Standard energy and efficiency corrections were performed for the SST. Energy spectra
from the ESA and SST merged into combined distributions were used to compute moments
and particle spectra. The moments were then corrected for the presence of oxygen (M3).
The presence of significant fluxes of relativistic electrons caused SST fluxes to
be saturated (M4) after 05:16UT, but that has no effect on the conclusions of our
study, which is focused on 04:40-05:00 UT (Fig 2; Extended Data Fig. 1). Distribution
function cuts discussed in a separate section (M5) are from ESA alone.
Data from the GOES 13 (G13) satellite are from the magnetometer
30
, medium-energy proton and electron, and high-energy proton and electron instruments
31
. Magnetometer data, obtained from the National Geophysical Data Center (NGDC), are
at 0.513s resolution. Protons between 95 and 575 keV are at 16.4 s resolution, and
those at 2.5 MeV are at 32.7 s resolution. Electrons 40 – 475 keV are at 60 s resolution;
those at 0.6 and 2 MeV are at 4.1 s resolution. Only the 40-475 keV electrons were
obtained as omni-directional, calibrated quantities; all others were retrieved as
uncorrected, directional fluxes and were averaged and rescaled to match levels published
online. Although absolute fluxes are only approximate (within a factor of 2), relative
flux changes, as used in this paper, are reliable.
The Auroral Electrojet (AE) Index
32
from the Kyoto World Data Center 2 (WDC2), at 1min resolution is also used. It denotes
the strength of ionospheric circulation, substorm activity, Joule heating and ionospheric
dissipation
4
.
The OMNI Database was used to retrieve the Disturbance Storm-Time (Dst) Index
32
(a measure of the total ring-current energy), at the same time resolution as from
WDC2. The Dst’s time rate of decrease is a measure of storm-time solar wind energy
conversion and deposition (dissipation) into the ring current. However, the raw index
also responds to magnetopause currents, which are unrelated to the ring current
33
. This effect can be corrected using the solar wind dynamic pressure. The pressure-corrected
Dst, denoted Dst*, was used (M8) to compute the total magnetospheric energy dissipation
4
.
Finally, we use data from the local horizontal, magnetic north component of the fluxgate
magnetometer at Bay Mills, MI (bmls)
34,35
, at 0.5s resolution.
M2. Coordinates
Geocentric solar magnetospheric (GSM) coordinates
36
were used for solar wind data (X: sunward; Y: cross-product of Earth’s magnetic dipole
axis and X; Z: completes the right-hand, orthogonal coordinate system). For nominal
(non-storm) conditions, the GSM system is also used in the magnetotail, as it adequately
describes the tail’s field line stretching and alignment with the Sun-Earth line (along
XGSM) far from Earth’s dipole
37
.
Under the storm-time conditions of our event, however, the model
15
field-line planes at THEMIS, as seen in Fig. 1A, were evidently rotated clockwise
about ZGSM towards the magnetic meridian; measurements also exhibit this behavior.
An appropriate system that is coplanar with the field lines prior to the onset of
reconnection was therefore used. Clockwise rotation by a common ϕrot ~10° angle about
the ZGSM axis at all satellites was found to minimize the absolute value of the By
component during the undisturbed time interval, 04:30-04:40UT, at P5 and P3 (the two
satellites straddling the current sheet but farthest from it at that time); this rotation
is also consistent with the model field-line planes (Fig. 1A). We formally defined
ϕrot as the angle between XGSM and the reverse of the instantaneous, time dependent,
average position of the THEMIS satellites (their geometric center), -R
GC, minus 10°. This allows us to analyze the reconnection phenomena in a natural coordinate
system defined by the plane of the field lines. We rotated all vector THEMIS data
(flows, fields, currents, distribution functions) accordingly. We refer to this rotated-GSM
system as XYZ throughout this paper and reserve the notation XYZGSM for the pristine
GSM coordinate system.
Closer to Earth (at and inside geostationary orbit), another system, the solar magnetospheric
(SM) system, in which the ZSM axis is exactly aligned with Earth’s magnetic dipole
36
, is used under nominal (non-storm) conditions, because a dipole is a reasonable approximation
of Earth’s field. In our storm-time event, however, the near-geostationary model
15
field lines at G13 are distorted considerably from dipolar (Fig. 1B): they are stretched
into a tail-like geometry by enhanced cross-tail currents. Hence, the SM system is
not useful to show the G13 data, but the GSM system is, so we use it instead. Additionally,
because G13 is near midnight (Fig. 1A), where meridional planes are expected to be
parallel to the XZGSM plane, no further rotation about ZGSM is needed to match field-line
planes. Indeed, when plotted in GSM, the G13 data (Fig. 2E, Extended Data Fig. 1F)
show a dominant Bx component increase (indicating current sheet thinning) just prior
to the interval of interest, Δt. No significant (out of plane) By was seen until near
or just after reconnection onset, again indicating that the field line planes as inferred
from the data are also approximately parallel to XZGSM.
In summary, we use GSM coordinates everywhere in this paper except for THEMIS, where
we use the rotated-GSM system rotated clockwise by ~10° about the ZGSM-axis to account
for the rotation of the field-line planes.
M3. O+ density fraction and associated plasma moment corrections
The measured total ion and electron densities differ consistently from each other
on all satellites (with Ni/Ne ~ 1/1.53), suggesting the presence of species heavier
than protons. This is because the ESA and SST ion instruments directly measure the
ion differential number flux at a given ion energy. Onboard software assumes that
this flux is made up of protons only. When a heavier ion species is present, its true
velocity is lower than derived by this instrument from the protons-only assumption
at each measured energy. The velocity moment is thus overestimated by the square root
of the heavier ion- to-proton mass ratio, resulting in a proportional underestimation
of the ion density
38
. During storms, the oxygen fraction f=NO+/Ne in the plasma sheet and the ring current
can be significant
39,40
. Assuming that O+ is the only species of significance other than protons, the measured
density
38
is: Ni/Ne = 1 − f + f/(mO+/mp)½. For the average observed ratio at P5 (similar to
that at P4), Ni/Ne ~ 1/1.53, we obtain f=0.46~0.5. Using this, we corrected the ion
velocity and the pressure tensor (both scaled upwards). Temperature and magnetic energy
per particle are unaffected by the presence of different species. Based on these densities,
the relevant species inertial lengths are: dO+~911km, dp+~228km, and de−~5km.
M4. SST saturation and background removal at P5
Prior to 05:16UT, fluxes of >35keV ions were below threshold, whereas fluxes of >35keV
electrons were not significant enough to affect the electron moments. In the 04:48-04:50UT
interval (around the time of X-line passage), increased background from >1MeV electrons
moving through the instrument walls (evident in both ESA and SST spectra), was removed
prior to moment computations by standard processing
29
, so it does not affect our study.
After 15:16UT the 35-300 keV electron fluxes increased considerably. SST electron
count rates exceeded 10ksamples/s. At these rates, background removal and dead-time
corrections cannot reconstruct the true fluxes, which are then regarded as a lower
estimate (saturated). However, ESA data are not saturated – standard ESA background
removal results in good quality lower-order moments (velocity, density) because those
are less dependent on energies measured by the SST. As a result of SST saturation
after 05:16UT, higher-order moments (electron/ion pressure and temperature) are underestimated
(e.g., over the double arrow in Extended Data Fig. 1J) and should be considered lower
limits during that period.
M5. Ion distribution functions
A representative ion outflow distribution at P5, tailward of the X-line at 04:47:29UT
(Extended Data Fig. 1L) and taken near the peak of the tailward flow period (Fig.
2G), demonstrates that the plasma sheet velocity moments contain two distinct populations:
a warm, dense, predominantly equatorward-moving beam (phase space density peaking
at Vz,gsm~ − 500km/s) and a hotter, tenuous, predominantly tailward-moving beam (peaking
at Vx,gsm ~ − 1500km/s), typical of reconnection outflows
41
. Embedded in a hot plasma sheet of lower density (green contours enveloping the two
beams in Extended Data Fig. 1L), these were moving equatorward at approximately the
same speed as the other two components. This is consistent with plasma sheet reconnection,
in which inflows and outflows (the first two components) are immersed in the ambient
plasma sheet plasma (the third component).
P4, farther from the neutral sheet than P5, based on its higher ∣Bx∣ value (Fig. 2C),
did not observe a strong tailward outflow but an inflow (Vz>0) into the reconnection
region (Fig. 2J). A representative ion distribution at P4 (Extended Data Fig. 1M)
shows two cold equatorward-flowing beams (flowing approximately perpendicular to the
magnetic field) separated by a factor of four in velocity (400km/s versus 100km/s)
embedded in a third, hot isotropic population, likely ambient plasma sheet ions as
seen at P5. The two cold beams are consistent with THEMIS’s total-ion instrument response
to two cold O+ and H+ populations, both ExB drifting at the same speed, ~100km/s.
Such a response is expected from the velocity transformation of energy bins into velocity
bins, under the incorrect assumption that all ions at a given energy are protons (M3);
it has been used in past studies for species discrimination on THEMIS
42
and other missions
43
. Also streaming parallel to the magnetic field (tailward), these beams are likely
of ionospheric origin. Projection of the total flow (perpendicular inflow and parallel
streaming) in the XGSM direction results in the tailward net flow (first moment of
the distribution) observed at this time. Although the third hot ambient plasma sheet
ion population is likely also a mixture of H+ and O+, it cannot be separated into
its constituent species because its velocity is too small compared to its thermal
velocity.
M6. Cross-tail current density (Jy), position, and thickness estimation
The average cross-tail sheet-like current (∂/∂z >> ∂/∂y, ∂/∂x=0) is approximately
Jy=μ0
−1(∂Bx/∂z - ∂Bz/∂x) ~ μ0
−1∂Bx/∂z. Applying the commonly used Harris model
44
, Bx(Z)/Blobe=tanh(Z-ZNS/LCS) yields: Jy(Z)= (μ0LCS)−1 Blobecosh−2(Z-ZNS/LCS), where
ZNS is the neutral sheet location and LCS the current sheet half-thickness. With Bx
measured at two satellites and Blobe estimated from the plasma and magnetic field
data (Fig. 2C), we can solve for the two unknowns, ZNS and LCS, and an estimate of
the peak current, Jy(ZNS)=μ0
−1Blobe/LCS. Of the two adjacent satellite pairs (P5-P4, P4-P3), the one that straddled
the neutral sheet typically produced the strongest current density estimate (Fig.
2D) and was used to obtain the current sheet parameters ZNS, LCS (Extended Data Fig.
1K).
We also estimated Jy,GSM=μ0
−1(∂Bx,GSM/∂z,GSM - ∂Bz, GSM/∂x,GSM) directly from the magnetic field measurements
on P3, P4 and P5 (without any other approximations), because those satellites are
located approximately on the XZ-GSM plane (their YGSM average separation from their
barycenter was only ~300km, whereas their XGSM and ZGSM average separations were 2500km
and 2900km, respectively). Outside 04:25-05:15UT, when the current sheet was thicker
than the inter-satellite separation, we obtained values that are nearly identical
to those from the Harris model. Inside that time interval, the Harris model provided
a larger current density estimate. This is understandable given that the Harris model
solution is: (i) a fit to an equation, not an average, and (ii) obtained from 2 satellites
at a smaller separation (δZ54~3300km, δZ43~3000km) than the 3-satellite linear dimension
(δZ53~6300km), and thus able to better resolve a current sheet of scale LCS⪅δZ54 (Extended
Data Fig. 1K) than the 3-satellite method.
M7. Hall system current density, JHS, estimation
The Hall system current for a thin sheet-like current (∂/∂z >> ∂/∂y, ∂/∂x ~ 0), JHS~Jx~
−μ0
−1∂By/∂z, comprises three current layers centered at the neutral plane: the middle
one near the equator and the north and south ones on either side of the equator (Fig.
1C, black open arrows). The equatorial layer current sheets are expected to flow towards
the reconnection site (across the magnetic field), whereas the off-equatorial layer
current sheets are expected to flow away from the reconnection site (nearly along
the field). We used the By measurements and inter-satellite Z-distance of the satellite
pair that straddled the neutral sheet (Extended Data Fig. 1K) to compute the equatorial
J
HS currents. We estimated the off-equatorial J
HS currents by assuming that the peak values of By were located halfway between the
peak and the minimum of the cross-tail current (i.e., between the current sheet center
and the current sheet boundary), LCS/2 away from the neutral sheet. If both paired
satellites were located farther than LCS/2 (the presumed By peak) from the neutral
sheet, δBy/δz was derived directly from the satellite By differences. If a pair straddled
the anticipated By peak, then the By average, <By>, was taken as a proxy of the peak;
the lobe By was assumed to be zero; and the spatial derivative was approximated as
<By>/(LCS/2). These assumptions resulted in a continuous estimate of the three current
sheet profiles thanks to the persistent Hall current system By signatures and the
continuous proximity of the satellites to the current sheet.
M8. Reconnection’s contribution to global flux and energy transport
Here we assess the reasonableness of the reconnection region’s size estimated from
the global-to-local flux and energy transport ratios integrated over the Δt~12min
interval (04:45-04:57UT) when reconnection was observed to be active at THEMIS.
The magnetospheric magnetic cumulative flux input is the solar wind electric field,
Ey,sw ~VtotBz,GSM (Extended Data Fig. 1C-D), applied across a nominal, 40RE cross
section of the tail at a 20% reconnection efficiency
45,46,21
, then time-integrated: Φin= 0.2·40RE·∫Ey,swdt (Extended Data Fig. 1E). Φin’s slope
(~1GWb/hr) multiplied by Δt gives ΔtΦin~0.2GWb.
The time-integrated flux transport per unit Y-distance in the magnetotail at P5 is:
Δt∫Eydt =<Ey>Δt ~11.5mV/m·12min ~0.053GWb/RE (<Ey> is the average Ey from Fig. 2H
over Δt). The ratio ΔtΦin/Δt∫Eydt provides an estimate of the tail reconnection region’s
effective width, ΔY~4RE. We call it effective because reconnection is impulsive and
likely localized; hence, it does not comprise a single X-line all across the active
region. This effective width is considerably smaller than the tail width (~40RE).
Thus, although intermittent and impulsive, the fast reconnection observed was effective
in producing the requisite storm-time flux transport, even if only operating over
a fraction of the tail width.
The solar wind energy input rate to the magnetosphere
4,1,46,21
is: ε[W]= (4π/μ0)VB2sin4(θ/2)·lo
2, otherwise known as the Akasofu “epsilon” parameter, with θ= acos(Bz,GSM/Byz,GSM),
lo =7RE. Its cumulative integral is: Uin= ∫εdt ~ 6.7PJ per hour (Extended Data Fig.
1E). Uin compares well (better than a factor of 2) with the cumulative integral of
the magnetospheric energy dissipation (also in Extended Data Fig. 1E) computed from
indices
4,1,46,21
, as: Umd=∫[4·1013(∂(−Dst*)/∂t + (−Dst*)/τR)+300·AE]dt ~ 4.1PJ per hour, where τR=1hr
is the ring-current decay rate of O+ through charge exchange (for AE and Dst* see
M1). We see that the magnetospheric energy dissipation during Δt is: ΔtUmd~0.82PJ.
The time-integrated Poynting flux, Sz=(ExBy-EyBx)/μ0, into the current sheet at P5
(using data from Fig. 2F, 2H) is: Δt∫Szdt=Δt∫[(ExBy-EyBx)/μ0]dt=<Sz>Δt ~0.78mW/m2·12min
~0.0228PJ/RE
2 (<Sz> is the 12min average of Sz). The ratio ΔtUmd/Δt∫Szdt provides an estimate
of the effective reconnection area, ΔXΔY= (0.82PJ)/(0.0228PJ/RE
2) ~36RE
2. Having already estimated ΔY~4RE, we obtain ΔX ~9RE, approximately ±30dO+ (or ±130dp+,
also reasonable), based on reconnection exhaust size estimates from simulations and
observations
18
. If centered at P5 when Vx, Bz reversed sign, the reconnection exhaust extends from
just earthward of G13 to ~X=−15 RE (the rectangle in Fig. 1A-B). This is consistent
with G13 observations of a strong Hall By.
M9. Equatorial Bz minimum: model versus data
The equatorial dipole field at P4 (Bz,dip~+28nT) alone is too large (compared to the
lobe field) to allow tail reconnection so close to Earth. It is suppressed, however,
by the Bz<0 contribution from the fringe field of the cross-tail current, which is
particularly strong during storms. The cross-tail current strength is regulated by
Pdyn as well as the amount of flux in the lobes (which is, in turn, controlled by
the solar wind southward magnetic field, Bz,GSM). Both Pdyn and negative Bz,GSM are
known to be enhanced during coronal mass ejections and stream-stream interaction regions
1
, the two main solar wind structure types leading to intense space storms. Therefore,
reduced near-Earth magnetotail equatorial Bz, may be a commonplace occurrence under
storm conditions. It is instructive to further examine evidence in our event that
an enhanced Pdyn and negative Bz,GSM resulted in favorable conditions for near-geostationary
reconnection.
Storm-time models of the magnetosphere, such as the TS0415 model (see field lines
drawn from it in Fig. 1A-B), parameterize current systems based on ensemble averages
of in-situ magnetic measurements. Because of the extreme thinning of the current sheet
during active times, such measurements are biased towards the plasma sheet boundary
layer, where field lines flare out of the neutral plane. The resultant models exaggerate
the negative Bz tail current contribution to the equatorial Bz profiles. Such models
can, however, provide useful estimates of the location of a local minimum in the equatorial
Bz, which is controlled by the equatorial distribution of the cross-tail currents.
Examination of the equatorial Bz profile in TS04 (Extended Data Fig. 3) for the solar
wind conditions just prior to reconnection onset in our event shows such a pronounced
minimum (−10 nT to −30 nT) across the tail at XGSM ~ −7.5 to −8.5 RE. Such Bz values
are, of course, unphysical: first, because reconnection should have occurred before
they were attained, and second, because even if they materialize after reconnection,
they are dynamically unstable due to JxB forces that will expel the resultant magnetic
loop on the tailward side further tailward, out to the solar wind. Such negative Bz
equatorial distributions, however, pinpoint the loci of anticipated equatorial Bz
minima, dictated by a cross-tail current distribution consistent with the magnetospheric
currents globally prescribed by the solar wind conditions at the time. In our event,
they show that reconnection was likely to occur anywhere across the equatorial magnetotail
(in Y) and near the X-distance where we observed reconnection onset, i.e., midway
between G13 and P3-5, at XGSM~ −8RE.
We next examine the observed magnetotail Bz just prior to reconnection onset (around
04:45:00-04:46:12UT), when LCS was less than the THEMIS inter-satellite separation
(Extended Data Fig. 1K). All satellites were farther from the neutral sheet than LCS,
as also evidenced by their large ∣Bx∣ (Fig. 2C, Fig. 2F, 2I). Because the Bz reduction
was partly due to field-line flaring, some modeling is needed to determine the equatorial
Bz. We use the simplest linear model of Bz’s falloff with distance from the equator
(Extended Data Fig. 4). We infer an equatorial field that was small (0.54±0.01nT,
<1% of Blobe) though still positive at the THEMIS satellites’ geometric center (XGSM~
−9.8RE). Such a Bz field is indeed small enough to overcome the stabilizing effect
of electron magnetization and permit reconnection under external forcing
47
. Even lower values of equatorial Bz may have been realized closer to Earth, at the
mid-point between G13 and P3, P4 and P5, resulting in yet another, spontaneous onset,
path to the tearing instability
48
.
Regardless of the specific kinetic process that may have caused current sheet tearing
and reconnection onset (important in its own right), the overarching condition that
led to the development of this Bz minimum is the solar wind driver -- in particular
its strong Pdyn and negative Bz,GSM. The equatorial Bz inferred is already so low
that small fluctuations in dynamic pressure could cause it to reconnect on timescales
faster than spontaneous tearing-mode growth rates
49,50
.
Extended Data
Extended Data Fig. 1.
Extended overview of reconnection region observations.
(A) Dst index encompassing several days around the event. (B) AE index (black, left
axis scale) and ground magnetometer magnetic pulsations from Bay Mills (bmls), band-pass
filtered at 10s-120s (northward component δBX shown, in blue, in the right vertical
axis scale; repeated from Fig. 2B for referencing the time of reconnection enhancement
around the time of enhancement in pulsation amplitude). (C) Solar wind magnetic field
at ARTEMIS P1 in GSM coordinates. In this and all subsequent panels showing vector
quantities, black, blue, green and red traces correspond to the vector magnitude,
and its X, Y, and Z components, respectively. Note that when not explicitly defined,
X,Y, Z components refer to the GSM coordinate system rotated about the ZGSM axis by
~10° to account for the approximate rotation of the field-line planes on THEMIS at
that time (M2). (D) Solar wind dynamic pressure, Pdyn (black); density, Ni (blue);
velocity magnitude, Vtot (red) also at ARTEMIS P1, showing that the Pdyn increase
was due to the Ni increase. (E) Cumulative integrals (M8) of: i) solar wind energy
coupling function ε, Uin= ∫εdt (black); ii) flux input rate in the magnetotail by
the solar wind electric field Ey,sw, Φin=0.2·40RE·∫Ey,swdt; and iii) magnetospheric
energy dissipation rate computed from Dst and AE, Umd (blue).(F) G13 magnetic field
components in GSM coordinates; (G) G13 proton fluxes at energies tabulated on the
right (increasing flux corresponds to decreasing energy); vertical blue arrows show
times of energization; (H) G13 electron fluxes corresponding to the energies tabulated
to the right as in (G); vertical red arrows show times of energization; (I) P5 magnetic
field components (shown for reference) in X, Y, Z rotated GSM coordinates; (J) ion
(Ti) and electron (Te) temperatures at P5 (saturation noted after 05:16UT causes temperatures
to be underestimated, but does not affect our conclusions (M4)); vertical blue and
red arrows correspond to ion and electron heating, respectively. (K) Estimate of ZGSM
location of the neutral sheet, ZNS (middle solid line), and current sheet thickness,
LCS (represented by distances of the upper and lower solid lines from the middle one),
obtained from Harris sheet model (M6), overplotted along with P3-P5 positions (colored
dashed lines); vertical lines are same as in Fig. 2E-J; they correspond to the interval
of interest (04:46:30 to 04:50:00UT) encompassing the fast flows (solid lines) and
time of the X-line passage (04:48:30UT) by P5 and P4 (dashed line). (L) Representative
ion velocity distribution function X-Z plane cut (X is positive to the left) during
one spin near the peak tailward reconnection outflows at P5, showing simultaneous
reconnection inflows from above the neutral sheet (M5). (M) Same as 1L but at P4,
at approximately the same time, showing reconnection inflows from below the neutral
sheet (M5).
Extended Data Fig. 2.
Correlation of flows, fields, and currents with Bx, Bz.
Quantities plotted compactly in Fig. 3 are shown here in raw format, plotted against
Bx, or Bz separately to reveal their correlation with these quantities, signifying
adherence to expectations from the reconnection paradigm and revealing the full excursion
of these quantities, which is obscure in color in Fig. 3. Quantities and units are
listed in horizontal axes; Bx or Bz are listed in vertical axes (common for left and
right panels). Different symbols correspond to various satellites in A-F and H and
to different distances from the neutral sheet in G (as denoted in inserts). Colors,
also representing satellites (P5/P4/P3 are magenta/blue/turquoise, respectively) in
A-D and F further help differentiate the sources of data. Colors represent sign of
Bz (red, blue for Bz<0, >0 respectively) in E, G and H. The time-interval plotted
is 04:46:30-04:50:00UT (vertical solid lines in Fig. 2) except for some deviations
for Vz, By, and Ez (denoted in the individual panels C, E and F), justified as follows:
for Vz (restricted to 04:46:30-04:48:30UT, tailward of the X-line), the earthward
side of the plasma sheet expanded lobeward, and the equatorward inflow (Vz) cannot
be cleanly separated from outward expansion; for By (at G13 only, restricted to 04:46:30-04:47:45UT),
the neutral sheet flapped southward (Extended Data Fig. 1K). and G13 moved closer
to the neutral sheet as its ∣Bx∣ was suddenly reduced (Fig. 2E) and all its components
became very noisy, presumably as it was immersed in the hot outflows from reconnection
– beyond 04:48:30UT the reconnection exhaust moved quickly away from G13 as the X-line
moved tailward; and for Ez (extended to 04:46:30-04:57:30UT) the interval is justified
by the persistence of the Hall system electric field at P5-3 over the entire interval
Δt (subsequent, secondary X-lines result in similar polarity Hall electric field,
towards the neutral sheet from both sides). To reduce clutter from random fluctuations,
low magnitudes of some quantities have been eliminated for Vx, By, Ez, and E∥, as
listed in the respective panels (A, E, F and H).
Extended Data Fig. 3.
Model equatorial Bz.
Equatorial Bz profile (color) from the TS04 model15 based on solar wind and Dst values
at 04:45UT, with field lines (solid lines: above magnetic equator; dashed: below)
and satellite locations from Fig. 1 superimposed (for reference). The magnetic equator
in the model was determined as the surface of
B
r
=
B
⋅
r
^
reversals, as a function of Z. At the equatorial (X, Y) projections of THEMIS satellites,
the model Bz ranges from −2.5 to −7nT.
Extended Data Fig. 4.
Equatorial Bz at THEMIS. Equatorial Bz at THEMIS.
Linear fit to Bz data from THEMIS P5-3 immediately prior to reconnection onset, 04:45:00
– 04:46:12 UT, as a function of their distance DNS from the neutral sheet position,
ZNS. The Harris sheet44 model was used to determine ZNS (M6). The inferred equatorial
Bz is ~ +0.54nT.