Azimuthal variation of magnetopause reconnection at scales below an Earth radius

by Ying Zou
Univ. of Alabama, Huntsville


Magnetic reconnection occurs as a magnetized plasma encounters another magnetized plasma with a shear in magnetic field, and the result is mixing plasmas and converting magnetic energy into kinetic energy. It accounts for many, if not most, explosive disturbances in space. Reconnection occurs at the Earth’s magnetopause, where it allows entry of solar wind energy into the magnetosphere, powering plasma convection, auroral displays, and geomagnetic storms. One important factor that controls the amount of the energy entering the magnetosphere is the structure of reconnection in the azimuthal direction. To understand how reconnection varies in this direction, efforts have been made to survey reconnection seen by spacecraft separated by various distances, and examine whether and how much the process differs. Among existing reports, the utilized spacecraft constellations are often too small or too large, only loosely constraining the range of variability. For example, at kinetic scales, signatures of reconnection are often simultaneously observed. On large scales of a few to >10 Earth radii (RE), the signatures are sometimes simultaneously observed, sometimes by one spacecraft only. The latter suggests that reconnection transitions from active to inactive at a-few-RE scale, although it is uncertain whether this scale reflects the true scale of variation, or merely gives an upper bound because of the spatial resolution of the observations offered by the spacecraft separation. Here, we use serendipitous constellations with a spatial size at a less explored scale size, which is few tenths of one RE, to explore the azimuthal variation of reconnection.

Figure 1. Panel a: spacecraft location in GSM Y-Z plane. The solid circle represents that the spacecraft observed signatures of reconnection, and the open circle represents that the spacecraft did not observe reconnection. Panel b: spacecraft location (blue dots) in the context of the magnetopause shear angle as viewed from the Sun. The shear angles were calculated with the magnetic field direction of the Tsyganenko 96 magnetic field model and the draped IMF conditions at the magnetopause. The IMF conditions were taken as 10-min averaged OMNI solar wind data. Locations of maximum shear angles are marked by the white curve. The black circle represents the size of the magnetopause at the terminator.


We utilize the constellation formed by three THEMIS spacecraft in the dayside science phase in Fall 2010 during which the spacecraft separation was a few tenths of RE in the azimuthal direction. We focus on situations where, according to reconnection signatures (e.g. fluid and kinetic), reconnection is present at one point and absent at another, and study the transition from presence to absence. One such event occurred on September 27, 2010. Figure 1 presents the locations of the three spacecraft in Geocentric solar magnetospheric (GSM) coordinates. As seen from Figures 1a, THE was positioned duskward of THA by only 0.1 RE, and THD was duskward of THE by 0.4 RE. Zooming out to the global scale, Figures 1b presents the spacecraft location in the context of the shear angle of the magnetic field lines between the sheath side and the magnetosphere side. The ridge of maximum magnetic shear is marked with the white curve and it signifies the most probable location for reconnection to occur. The three spacecraft (marked as blue dots), whose locations became indistinguishable given the large axis ranges of this figure, were positioned close to the reconnection line.

Panels a1-a4, b1-b4, and c1-c4 in Figure 2 present measurements of magnetic field, plasma density, ion energy spectra, and ion bulk velocity. These measurements are used to discern the presence/absence of reconnection jets, which is the fluid signature of reconnection. THD observed an ion bulk flow with a peak speed of 357 km/s (Panel c4). To assess whether this observed bulk flow is consistent with being a reconnection jet, we perform the Walén test. The Walén relation predicts how the tangential velocity component changes across a rotational discontinuity. If the bulk velocity moment is >50% consistent with the Walén prediction, it is labeled as an active reconnection jet. The Walén test reveals that when projecting the observed velocity onto the predicted jet direction, the observation was 83% of the prediction. THE also detected a bulk flow, whose projection was 71% of the Walén prediction. On the other hand, the ion bulk flow at THA was only 39% of the Walén prediction, implying that the reconnection jet subsided between THE and THA.

Figure 2. Panels a1-a4, b1-b4, and c1-c4: magnetic field, plasma density, ion energy spectra, and ion bulk velocity at THA, THE, and THD, respectively. The magnetic field data were taken from high-resolution FGM data for THA and THD, and from low-resolution FGM data for THE. The plasma measurements were taken from burst mode ESA data for THA, and THD, and reduced mode for THE. The magnetic field and velocity are presented in LMN coordinates, which are determined by minimum variance analysis of each spacecraft data. Panels a5, b5, and c5: ion distributions at THA, THE, and THD, respectively, as taken from ESA data. These distributions are taken at the inner edge of the low latitude boundary layer, which is marked by the black vertical lines in Panels a1-a4, b1-b4, and c1-c4.

Panels a5, b5, and c5 in Figure 2 present snapshots of ion distribution functions made by the three spacecraft. The snapshots were taken at the inner/earthward edge of the magnetopause boundary layer, whose locations are marked as the black vertical lines in Panels a1-a4, b1-b4, and c1-c4. The ion distribution functions are presented in bulk velocity-magnetic field plane, and the small red line extended from the origin indicates the direction and speed of the bulk velocity moment of the distribution. A dashed black circle marking a speed of 400 m/s is overlain for the ease of comparison. All three snapshots show the existence of two distinct populations, the dense and cold magnetospheric ions with a low speed, and the more tenuous and hotter magnetosheath ions. The speed of the magnetosheath ions decreases from THD, to THE, and to THA, similarly to the bulk flow speeds seen in Panels a4, b4, and c4. The speed at THA was significantly lower than the Walén prediction, which was 417 km/s at the given spacecraft location in the spacecraft reference frame. This suggests that the amount of acceleration of magnetosheath ions associated with reconnection decreased from THD to THA. The cold magnetospheric ions at THA had a substantial velocity perpendicular to the magnetic field (Panel a5), possibly resulting from the motion of the magnetopause.


We examine whether reconnection at the Earth magnetopause varies at tenths of RE in the azimuthal direction by inspecting the plasma bulk flow and ion distribution functions. The result shows that the amount of plasma acceleration, compared with the Walén prediction, can decrease substantially over tenths of RE. Although not shown, we do not find causes of such variation based on local magnetopause or upstream magnetosheath conditions. Such a finding indicates that the tenths of RE scale is a critical azimuthal scale for magnetopause reconnection, at least during certain stages of its development.


Zou, Y., Walsh, B. M., Atz, E., Liang, H., Ma, Q., Angelopoulos, V. (2020). Azimuthal variation of magnetopause reconnection at scales below an Earth radius. Geophysical Research Letters, 47, e2019GL086500. https://doi.org/10.1029/2019GL086500

Biographical Note

Ying Zou is an assistant professor at University of Alabama in Huntsville. Her research interests focus on the interaction between the solar wind and the Earth’s magnetosphere and ionosphere.

Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu