2024 THEMIS SCIENCE NUGGETS
The Response of the Earth Magnetosphere to Changes in the Solar Wind Dynamic Pressure: I. Ion and Electron Kappa Distribution Functions.
Eyelade Adetayo Victor
Departamento de Fisica, Facultad de Ciencias, Universidad de Chile, Santiago, Chile
Introduction
The Earth’s magnetosphere is like an invisible shield that protects our planet from the constant powerful stream of charged particles flowing from the Sun, known as the solar wind. This invisible shield, created by Earth’s magnetic field, acts like a protective bubble, that deflects most of the harmful radiation and energetic particles that would otherwise have a negative impact on humans on Earth. However, the interaction between the solar wind and the magnetosphere is complex. In space, particles don’t collide; instead, they follow the influence of electric and magnetic fields in unique ways. Scientists use special mathematical models known as Kappa distributions, to describe how these particles behave in space plasma environment. These models explain why some particles have much more energy than expected, which form high-speed “tails” in their distribution. By fitting the Kappa distributions model to ions and electrons flux spectra, we obtain key parameters that describe the particles behavior. One important parameter, the kappa-index (k), allows us to understand the energetic tails of these distributions. Considering that changes in solar wind pressure cause changes in ions and electrons distribution. We analyze data from the five THEMIS satellites to investigate these changes. We focus on the kappa-index for ions and electrons to better understand their behavior and what they reveal about plasma environments, such as the Earth’s magnetosphere.
Results
Using data from NASA’s THEMIS satellites, we studied how the solar wind dynamic pressure affects ion and electron particle distributions. In Figure 1, we illustrate how the ion kappa-index (κi) varies across different Magnetic Local Time (MLT) regions in response to changes in solar wind pressure (PSW) as shown by the PSW intervals given on top of each panel of Figure 1. We observe that κi depends on both the MLT and distance from Earth. In general, we found that ki decreases as PSW increases for all values of MLT. This suggest that when PSW is low, the plasma exhibit high kappa values (Maxwellian distribution). In contrast, there are more extra energetic particles (suprathermal distributions) for high PSW. For all ranges of PSW, the minimum values of κi are observed between 5 and 12 MLT, meanwhile the maximum values are concentrated at the night side between 22 and 5 MLT. However, at high PSW, the κi distribution becomes more uniform, except for a noticeable peak between 22 and 24 MLT. Another key feature is a narrow ring-like structure with low κi values, appearing at different locations in the Dawn, Dusk, and Midnight sectors, around 12–13 Earth radii (RE). This structure separates the plasma into two distinct regions, as highlighted by the black arrows in Figure 1b, and is especially pronounced when PSW exceeds 1 nPa.
To better understand these effects, we separate our data into four different regions based on MLT: dawn (4–8 MLT), noon (10–14 MLT), dusk (16–22 MLT), and midnight (22–2 MLT). We then plot how the κi changes with distance from Earth in each region as shown in Figure 2. The different MLT regions are represented by different colors, as shown in the inset of the figure, and the error bars indicate the standard error. Our results reveal clear patterns across the MLT regions. At noon, κi increases from 7 Earth radii (RE) to about 10 RE, then decreases at greater distances. A similar trend is seen in the dawn, dusk, and midnight regions, but with a noticeable drop in κi between 12 and 13 RE. This drop corresponds to a narrow, ring-like structure where κi reaches its lowest value. Beyond 13 RE, κi starts increasing again. Figure 2 also shows that at nearly every distance and in almost every MLT sector, higher solar wind pressure leads to lower κi values (red points appear below black points in the plots).
Figure 3 shows the results obtained for electron kappa-index (ke) for the same ranges of PSW. We found that the behavior of the κe is very different for the day and night MLT sectors. We have higher values (more Maxwellian) of κe at the dawn, dusk, and night sectors than at the day side. This behavior differs from the ions, where κi become more symmetric as PSW increases. As in the case of the ions, we can see a separation of two plasma domains in the values of κi. However, while κi exhibits a minimum, κe exhibits a maximum, at the same range of distances. In contrast to κi, in the night sector κe increase and decrease with radial distance and exhibit a maximum at approximately the same radial distance where κi has a minimum. If we analyse the overall behavior of the κe with the PSW, we can observe that the κe averaged over all radial distances at each MLT sector as seen in Figure 4 also increases with PSW while for κi decreases with increasing PSW. Overall, when considering the solar wind dynamic pressure, it was found that ki and ke exhibit changes that correlate with PSW variations in different fashions, depending on the MLT sector.
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| Figure 1. Color-coded maps showing how the ion kappa index (κi) varies with different solar wind pressures and magnetic field strengths. The black arrows in (b) highlight a distinct ring-shaped pattern that separates two different plasma regions. |
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| Figure 2. Radial profile of ion kappa index (κi) for IMF components in the range ±5 nT and different ranges of solar wind dynamic pressure as indicated on top of each panel. |
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| Figure 3. Color-coded maps showing how the electron kappa index (κe) varies with different solar wind pressures and magnetic field strengths. |
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| Figure 4. Radial profile of electron kappa index (κe) for IMF components in the range ±5 nT and different ranges of solar wind dynamic pressure as indicated on top of each panel. |
Conclusion
We examined how the ion and electron kappa parameters in the Earth's magnetosphere, obtained using THEMIS data, respond to solar wind dynamic pressure conditions. We found that for all MLT sectors and all radial distances ranging between 7 and 15 RE, or up to the magnetopause, the ion kappa-index (κi) decreases as solar wind dynamic pressure (PSW) increases. κi also shows a clear MLT dependence with the maximum values at the night side, and the minimum values in the late dawn sector. On the other hand, the electron kappa index (κe) is almost independent of PSW in the flanks. It has smaller values in the noon, and it takes larger values in the night sectors, generating a strong day‐night asymmetry. Interesting narrow partial ring‐shaped structures extending over different azimuthal ranges are observed as a local minimum for κi, and as a local maximum for κe.
References
Eyelade, A. V., Stepanova, M., Espinoza, C. M., Antonova, E. E., and Kirpichev, I. P. (2024). The response of the magnetosphere to changes in the solar wind dynamic pressure: 2. Ion and electron kappa distribution functions. Journal of Geophysical Research: Space Physics, 129, e2023JA031949. https://doi.org/10.1029/2023JA031949Biographical Note
Adetayo Victor Eyelade is currently a postdoctoral fellow at the Department of Physics, Universidad de Chile in Santiago. He earned his Ph.D. in Space Physics from the Department of Physics, Universidad de Santiago de Chile in 2021. His research focuses on non-equilibrium plasmas, particularly in experimental and observational space plasma physics. He investigates the role of Kappa distributions on the microscopic and macroscopic scales. His work explores plasma dynamics in space environments such as Earth's magnetosphere and the solar wind, using data from THEMIS spacecraft missions.
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