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# Near-Earth magnetotail reconnection powers space storms

## Abstract

Space storms1 are the dominant contributor to space weather. During storms, rearrangement of the solar wind and Earth’s magnetic field lines at the dayside enhances global plasma circulation in the magnetosphere2,3. As this circulation proceeds, energy is dissipated into heat in the ionosphere and near-Earth space. As Earth’s dayside magnetic flux is eroded during this process, magnetotail reconnection must occur to replenish it. However, whether dissipation is powered by magnetotail (nightside) reconnection, as in storms’ weaker but more commonplace relatives, substorms4,5, 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 of ~6.6–10 Earth radii—probably 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 towards space weather prediction.

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## Data availability

THEMIS and ARTEMIS data are available through http://themis.ssl.berkeley.edu. GOES data were accessed from https://www.ngdc.noaa.gov/stp/satellite/goes/dataaccess.html.

## Code availability

THEMIS and ARTEMIS mission data (including ground magnetometer data), GOES 13 data and OMNI data have been imported, analysed and plotted using corresponding plug-ins to the SPEDAS analysis platform (http://spedas.org; ref. 24).

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## Acknowledgements

This work was funded by NASA contract NAS5-02099. We thank M. Bester and the THEMIS operations team for the continuing sound operation of the five satellites, and the instrument leads: J. W. Bonnell and F. S. Mozer (EFI); D. Larson (SST); C. W. Carlson and J. P. McFadden (ESA); K. H. Glassmeier, U. Auster and W. Baumjohann (FGM); S. Mende and C. T. Russell (ground magnetometers). We thank the analysis software teams for TDAS and SPEDAS. We also thank the National Center for Environmental Information for use of the GOES 13 data, specifically H. J. Singer for the use of the magnetometer data and T. Onsager and J. Rodriguez for the use of the particle instrument data, both accessed via SPEDAS plug-ins. We acknowledge use of NASA/GSFC’s Space Physics Data Facility’s OMNI data services, accessed via a SPEDAS plug-in. We are grateful to J. Hohl and E. Masongsong for their editorial assistance.

## Author information

Authors

### Contributions

V.A. conducted project planning, data analysis, interpretation and manuscript preparation; A.A. contributed to theory and interpretation; T.D.P. contributed to data analysis and interpretation; Y.M. contributed to interpretation and relationship to past studies of reconnection.

### Corresponding author

Correspondence to Vassilis Angelopoulos.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature Physics thanks Raluca Ilie and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## 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 ~10o to account for the approximate rotation of the field-line planes on THEMIS at that time (also see Methods, Coordinates). 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 (see Methods, Reconnection’s contribution to global flux and energy transport) 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 (also see Methods, SST saturation and background removal at P5)); 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 (Methods, Cross-tail current density (Jy), position and thickness estimation), 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 (also see Methods, Ion distribution functions). m, Same as 1L but at P4, at approximately the same time, showing reconnection inflows from below the neutral sheet (also see Methods, Ion distribution functions).

### 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 af 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 $$B_r = B\hat r$$ reversals, as a function of Z. At the equatorial (X, Y) projections of THEMIS satellites, the model Bz ranges from −2.5 to −7 nT.

### 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 (see Methods, Hall system current density, JHS, estimation). The inferred equatorial Bz is ~ +0.54 nT.

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Reprints and Permissions

Angelopoulos, V., Artemyev, A., Phan, T.D. et al. Near-Earth magnetotail reconnection powers space storms. Nat. Phys. 16, 317–321 (2020). https://doi.org/10.1038/s41567-019-0749-4

• Accepted:

• Published:

• Issue Date:

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