2025 ARTEMIS SCIENCE NUGGETS

Global structure of the cislunar magnetotail and its evolution during substorms

by Mikhail Sitnov
The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland


Introduction

An empirical description of the structure and dynamics of the Earth's nightside region, where the magnetic field lines are strongly stretched in the antisunward direction forming the magnetotail, is very important for understanding the global magnetospheric convection, its space weather effects, magnetic storms and substorms, and in view of the future missions to the Moon. Meanwhile, in its outer part, beyond half the distance to the Moon, which completes the cislunar magnetotail, it is poorly investigated. Here, we apply modern data mining (DM) techniques to reconstruct the magnetic field and electric current distributions in the cislunar tail using data from many missions since 1995, including observations made by Geotail, IMP-8 and particularly ARTEMIS [Angelopoulos, 2011] in its outer part.

Results

The main challenge in the global empirical description of the magnetospheric magnetic field and underlying electric currents is an extreme paucity of in-situ observations with less than a dozen of spaceborne magnetometers available at any moment. To meet this challenge, it was proposed [Sitnov et al., 2008; Stephens et al., 2019] to mine multi-mission and multi-decade archives of the magnetic field. This DM approach takes into account the recurring nature of storms and substorms reflected in similar patterns of the corresponding activity indices. It also takes into account the solar wind driving of the magnetosphere. It allows one to extract from the historical database a small (<1%) subset of observations made when the solar wind driving, storm and substorm states of the magnetosphere were most similar to the moment of interest. Since the total amount of data accumulated in the database may be quite large (~107 points with 5-min cadence for the period since 1995 [Stephens et al., 2019; Sitnov et al., 2025]), even their ~1% part should contain much more data points (~105) than the number of actual spacecraft available at the moment. Such a large number of synthetic probes allows one to employ very complex and flexible magnetic field architectures with many (up to ~103) free parameters [Tsyganenko & Sitnov, 2007; Stephens et al., 2019], making it possible to resolve the complex 3-D structure of storm and substorm currents.

The global structure of the cislunar tail turned out to be very interesting and unexpected. Earlier, local ARTEMIS observations using tail current sheet (CS) flapping motions [Kamaletdinov et al., 2024] suggested that lunar CSs may be very thin and intense: About 56% of flapping CSs have thicknesses comparable to the ion gyroradius km within a field B0 ~ 10 nT outside the sheet. At the same time, CSs with current densities from 1 to 10 nA/m2 have been observed in 64% of cases. If such thin CS extended from the near-Earth region, their appearance at lunar orbit would strongly violate the isotropic force balance. It suggests that the CS aspect ratio Lz/Lx ~ Bz/B0. Therefore, at the lunar distance Lz/Lx ~ 1000 km / 60 RE ~ 0.003, and the isotropic balance requires Bz/B0 ratios to be much smaller compared to their observed quiet-time values ~0.1, according to [Runov et al., 2023]. Therefore, thin cislunar CSs must be overstretched, that is, the iso-contours of the current density should be stretched more than the field lines.

Figure 1. Global structure of the cislunar magnetic field in the GSW coordinate system presented in the form of (a) the distribution of the current density and sampled magnetic field lines in the meridional plane and (b) the equatorial distribution of the magnetic field Bz (with the tilt angle and the twisting parameter taken to be zero for simplicity) at the onset of the January 19, 2019 substorm [Sitnov et al., 2025]. Overplotted dots show the corresponding DM subset of 113,065 points used for reconstruction, red for ARTEMIS (3,496 points) and gray for other missions.

DM reconstructions [Sitnov et al., 2025] have confirmed that the tail CSs are indeed overstretched (Figure 1a) but they unlikely reach the lunar distance. Strong currents found in the DM reconstructions beyond 31 RE and even at lunar distance resemble currents found in local ARTEMIS observations and are close to them in the current density magnitude. However, thin and intense CS are seen in the cislunar tail reconstructions largely near the O-lines of the corresponding plasmoids or flux ropes (e.g., Figs. 7 and 13 in [Sitnov et al., 2025]).

One more interesting feature of the cislunar tail revealed by DM-reconstructions are the bumpy profiles of the equatorial Bz field in the presence of the solar wind/IMF loading (Figures 1b and 2). While before and after the loading periods (see, for instance, black lines in Figures 2c and 2d) these profiles are relatively smooth and monotonic, in active loading periods (BzIMF<0) they have one or more bumps. Such bumps are already known within 31 RE due to both DM reconstructions [Stephens et al., 2019] and remote sensing using isotropic precipitation boundaries [e.g., Shi et al., 2024]. The cislunar reconstructions show similar bumps in the substorm growth phase (around 19 RE in Figure 2a and 16 RE in Figure 2c). However, they also suggest that there may be more than one bump or Bz minimum in the equatorial magnetic field profile. Moreover, the bumps beyond 31 RE (e.g., around 37 RE in Figure 2a and 32 RE in Figure 2c) may be more pronounced, being likely pumped by magnetic reconnection near ~40 RE (Figure 1b).

Figure 2. The equatorial magnetic field Bz sampled along the line -65 RE < x < -7.5 RE during the January 19, 2019 (a-b) and January 11, 2017 (c-d) substorms [Sitnov et al., 2025].

Conclusion

The data mining reconstructions of the cislunar magnetotail using ARTEMIS and other mission data show regular distributions of the magnetic field and current density, consistent with the superposed epoch analysis, statistical studies of traveling compression regions and plasma flows. The distributions also regularly evolve during substorms both in the near-Earth region, where their evolution is consistent with earlier reconstructions [e.g., Stephens et al., 2019], and in the outer half of the cislunar tail, where the magnetic field evolution can be verified by in-situ ARTEMIS observations. The mismatch between ARTEMIS observations and the DM magnetic field also suggests substantial deviations from this mean picture, which can be caused by flapping motions, plasma turbulence, fast plasma flows and unresolved global processes, such as spontaneous magnetic reconnection. The reconstructions also reveal large areas of the magnetotail that are very sparsely populated by in-situ observations (Figure 1), particularly, the gap region between 31 and 55 RE, whose structure is the most intriguing but is resolved mainly through extrapolation of the neighboring regions, very few measurements inside the gap and the high efficiency of the DM method. So, the present analysis should stimulate future cislunar tail mission projects and remote sensing observations.

References

Angelopoulos, V., The ARTEMIS mission, Space Science Reviews, 165(1), 3–25, 2011

Kamaletdinov, S. R., Artemyev, A. V., Runov, A., & Angelopoulos, V., Characteristics of thin magnetotail current sheet plasmas at lunar distances, Journal of Geophysical Research: Space Physics, 129(8), e2024JA032755, 2024

Runov, A., Angelopoulos, V., Khurana, K., Liu, J., Balikhin, M., & Artemyev, A. V., Properties of quiet magnetotail plasma sheet at lunar distances, Journal of Geophysical Research: Space Physics, 128(11), e2023JA031908, 2023

Shi, X., Stephens, G. K., Artemyev, A. V., Sitnov, M. I., & Angelopoulos, V., Picturing global substorm dynamics in the magnetotail using low-altitude ELFIN measurements and data mining-based magnetic field reconstructions, Space Weather, 22(10), e2024SW004062, 2024

Sitnov, M. I., Tsyganenko, N. A., Ukhorskiy, A. Y., & Brandt, P. C., Dynamical data-based modeling of the storm-time geomagnetic field with enhanced spatial resolution, Journal of Geophysical Research, 113(A7), A07218, 2008

Sitnov, M. I., Stephens, G. K., Artemyev, A. V., Motoba, T., & Tsyganenko, N. A., Global structure of the cislunar magnetotail and its evolution during substorms, Journal of Geophysical Research: Space Physics, 130, e2025JA034018, 2025

Stephens, G. K., Sitnov, M. I., Korth, H., Tsyganenko, N. A., Ohtani, S., Gkioulidou, M., & Ukhorskiy, A. Y., Global empirical picture of magnetospheric substorms inferred from multimission magnetometer data, Journal of Geophysical Research: Space Physics, 124(2), 1085–1110, 2019

Tsyganenko, N. A., & Sitnov, M. I., Magnetospheric configurations from a high-resolution data-based magnetic field model, Journal of Geophysical Research, 112(A6), A06225, 2007

Biographical Note

Mikhail Sitnov is a Principal Professional Staff at the Johns Hopkins University Applied Physics Laboratory. He earned his PhD from the Physics Department of the Moscow State University in 1986. His research is focused on data mining and data assimilation with applications to the empirical reconstruction of the geomagnetic field, kinetic theory and simulations of magnetic reconnection, magnetotail instabilities and nonlinear plasma structures.

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