2021 THEMIS SCIENCE NUGGETS
Magnetic Field in Magnetosheath Jets: A Statistical Study of Bz Near the Magnetopause
by Laura Vuorinen
Department of Physics and Astronomy, University of Turku, Turku, Finland
Introduction
Magnetosheath jets are dynamic pressure enhancements downstream of the Earth’s bow shock. They have been linked to the quasi-parallel shock, as they are mostly observed close to the shock and during low IMF cone angle conditions in the dayside magnetosheath. Some jets propagate deep into the magnetosheath and eventually hit the magnetopause with high dynamic pressure. Observations imply that these impacts can launch surface waves on the magnetopause and trigger magnetopause reconnection. These effects can go on to manifest themselves as disturbances in the magnetosphere and the ionosphere. As jets are estimated to be frequently hitting the magnetopause, statistical studies are needed to better understand their role in the solar wind-magnetosphere coupling. In this study, we focus on the possible effects of jets on magnetopause reconnection.
Magnetic reconnection is one of the most important space weather processes. At the magnetopause it connects the interplanetary magnetic field (IMF) with the Earth’s magnetic field and allows the entry of solar wind plasma and energy into the magnetosphere. Magnetic shear at the magnetopause is a controlling parameter for reconnection, which is efficiently driven when the IMF has a southward component (GSM Bz < 0) anti-parallel to the Earth’s northward field (Bz > 0). Jets have been proposed to locally affect magnetopause reconnection by altering the local magnetic field and plasma conditions. To understand how likely it is for jets to statistically affect reconnection, our first approach is to determine the typical magnetic field Bz orientation of jets near the magnetopause.
Results
We statistically study jets using THEMIS spacecraft magnetosheath data from the years 2008–2011 together with OMNI solar wind data. We compare the Bz distribution in jet intervals to that in similar-duration non-jet intervals, separately during northward and southward IMF. The intervals have been sampled during similar IMF cone angle conditions. We are particularly interested in how common Bz of opposite polarity to the IMF Bz polarity is within the intervals. In Figure 1, we show a time series of THEMIS D measurements from the magnetosheath including an example jet. We use the jet definition by Plaschke et al. (2013), of which main criteria are: (1) the earthward dynamic pressure within a jet has to exceed half of the solar wind dynamic pressure at some point, and (2) within the whole jet interval it has to be larger than a quarter of the solar wind dynamic pressure. In this particular jet, the magnetic field alternates from northward to southward with a wave-like pattern. The longest northward and southward Bz periods within the jets are shaded in orange and magenta, respectively.
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| Figure 1. Time series of THEMIS D magnetosheath measurements with an example jet. The panels include: (a) magnetic field in GSM coordinates, (b) OMNI IMF data in GSM coordinates, (c) number density, (d) velocity in GSM coordinates, (e) earthward dynamic pressure (black) and OMNI solar wind dynamic pressure (1x: blue, 1/2x: red, 1/4x: green), and (f) ion energy flux density. The jet interval is highlighted in purple shading and the longest periods of northward and southward Bz within the jet interval are highlighted in orange and magenta, respectively. |
In Figure 2, we statistically investigate these longest periods of southward Bz during northward IMF (Figure 2a) and northward Bz during southward IMF (Figure 2b) in jet (blue) and non-jet intervals (red). Around 40% of non-jet intervals have some opposite Bz within them, while around 60-70 % of jets exhibit opposite Bz. These periods of opposite Bz in jets tend to be shorter than those in non-jet intervals, as their median durations are around 10 s and 20 s, respectively. However, as these periods are more common in jets, we find that during northward IMF, southward periods up to 22 s (12 s as a conservative lower estimate) are more common within jets. Similarly, during southward IMF northward periods up to 14 s (7 s as a conservative estimate) are more common within jets.
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| Figure 2. The durations of the longest periods of (a) southward Bz during northward IMF and (b) northward Bz during southward IMF within jet (blue) and non-jet (red) intervals. The data is from the region F ∊ [-0.1, 0.3) from the relative radial distance scale between the Shue et al. (1998) model magnetopause (F = 0) and the Merka et al. (2005) model bow shock (F = 1), i.e., close to the magnetopause. The distributions are presented as cumulative distribution functions (CCDFs, i.e., the percentages of observations above a certain duration) that also contain the 0 s durations. The CCDFs are the means of 500 samples and the highlighted regions are the 95% confidence intervals. Durations of 10 s and 30 s are also separately marked. |
Conclusion
We investigate the magnetic field component Bz in magnetosheath jets to understand whether jets could statistically affect magnetopause reconnection via their magnetic fields. As a background, 40 % of non-jet magnetosheath intervals exhibit some Bz opposite to the prevailing IMF Bz given by OMNI data, e.g., southward Bz during northward IMF and vice versa. On top of this, 60–70% of jet intervals exhibit some opposite Bz. Thus, pulses of opposite Bz are more common in jets, and jets introduce southward Bz to the magnetopause during northward IMF and northward Bz during southward IMF. Consequently, jets may have potential to locally affect magnetopause reconnection, e.g., trigger it during northward IMF and suppress it during southward IMF. However, these periods of opposite Bz tend to be short, as the median duration is 10 s. The significance of these effects depends on what kind of perturbations can affect the local state of reconnection. Future studies of jet-related reconnection events are needed to answer this question.
Reference
Plaschke, F., Hietala, H., and Angelopoulos, V.: Anti-sunward high-speed jets in the subsolar magnetosheath, Ann. Geophys., 31, 1877–1889, https://doi.org/10.5194/angeo-31-1877-2013, 2013.Vuorinen, L., Hietala, H., Plaschke, F., and LaMoury, A. T.: Magnetic field in magnetosheath jets: A statistical study of Bz near the magnetopause. Journal of Geophysical Research: Space Physics, 126, e2021JA029188, https://doi.org/10.1029/2021JA029188, 2021.
Biographical Note
Laura Vuorinen is a PhD student in space physics at the University of Turku, Finland. Her research focuses on the occurrence and effects of high-speed jets in the Earth’s magnetosheath.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu