James Plank
Gather.town id
MIS10
Poster Title
The evolution of turbulence through the transition region of Earth's quasi-parallel bow shock
Institution
University of Southampton
Abstract (short summary)
Simulations and observations have recently shown that reconnecting current sheets are present in the transition region of Earth's bow shock. Current sheets are also observed in the magnetosheath as a consequence of turbulent reconnection. However, the link between shock and magnetosheath reconnection is not yet known. To investigate this link, we use observations of a crossing of Earth's quasi-parallel bow shock by Magnetospheric Multiscale (MMS) on 18/04/2020. For this event, we characterise magnetic fluctuations as the spacecraft crosses the extended transition region of the shock, and continues downstream into the magnetosheath. Making use of the Fluxgate-Searchcoil Merged dataset, the ~11 minute burst interval is split into consecutive or sliding windows of ~3.7s each. We quantify changes in fluctuations across the transition region; including in the slope of the magnetic spectrum, the scales of breaks in spectral power laws, and the correlation length of the fluctuations. Furthermore, we examine how these quantities change across the inertial, ion and electron scales. This provides insight into the evolution of turbulence and reconnection in the solar wind, through the shock transition layer, and then towards the magnetosheath.
We find that the ion scale magnetic spectra remain minimally affected through the shock, while strong steepening of the spectra at electron scales may be linked to changes in dissipation mechanisms at those scales, such as onset of electron-only reconnection. Before the shock, in the solar wind, the slopes of the magnetic spectra are steepest in the ion range, yet at the shock crossing, the electron scale spectrum steepens considerably to match the ion slope. Furthermore, the spectral break between ion and electron scales becomes much less significant.
We find that the ion scale magnetic spectra remain minimally affected through the shock, while strong steepening of the spectra at electron scales may be linked to changes in dissipation mechanisms at those scales, such as onset of electron-only reconnection. Before the shock, in the solar wind, the slopes of the magnetic spectra are steepest in the ion range, yet at the shock crossing, the electron scale spectrum steepens considerably to match the ion slope. Furthermore, the spectral break between ion and electron scales becomes much less significant.
Plain text (extended) Summary
The evolution of turbulence through the transition region of Earth's quasi-parallel bow shock.
Authors: James Plank (jp5g16@soton.ac.uk) and Imogen Gingell
1. INTRODUCTION: Reconnection has been observed at Earth’s bow shock, and has also been observed to influence properties of turbulence. At shocks, reconnection can occur in a disordered/turbulent transition region. Observations from Magnetospheric Multiscale (MMS) have been used to identify an event (Fig. 1) that shows in detail how the magnetic spectrum evolves as solar wind plasma crosses a quasi-parallel bow shock and enters the transition region downstream of the shock.
2. POWER LAWS AT DECREASING SCALES: The magnetic spectrum (Fig. 2, 3) displays power law relationships at inertial, ion and electron scales. The steepness of the slope both in relative (compared to smaller/larger scales) and absolute terms is related to turbulent energy cascades.
Fig. 2: Magnetic spectrum in STR (6.7s window). There is a distinct change in power law slope visible at ~ion scale, however, there is no clear change at the electron scale. Slope stays flat from ~ion limit almost down to instrument limit.
Fig. 3: The magnetic spectrum in SW. Multiple power law slopes are visible. Two power laws have been identified by MARS algorithm in the ion range. There is also a separate power law visible for electron scales.
3. EVOLUTION OF SPECTRUM ACROSS SHOCK: By splitting the event into ~6.7s intervals (two examples Fig. 1 & Fig. 2), we can show how power law slopes change for each range of scales.
METHOD A: Assuming spectral breaks occur at k=rho i and k=rho e=de, the slope associated with each region can be calculated with a linear regression (Fig. 4 middle).
METHOD B: Using the MARS algorithm we chain (an arbitrary number of) linear fits end-to-end across the entire spectrum -3.6≤ log k ≤1 (Fig. 4, bottom). The spectral break is not guaranteed to be placed exactly on the ion/electron gyroradius (see Fig. 2 & 3). Hence, this results in a more accurate fit to the magnetic spectrum.
Fig. 4: Middle: Slope for inertial, ion and electron ranges, as calculated by method A. Inertial range slope flattens across the shock. Ion slope does not change significantly. Electron range slope becomes steeper. Bottom: Time dependence of best fit power laws to magnetic spectrum identified by method B (MARS). As plasma enters shock transition region, the distinction between electron and ion ranges is lost.
4. COMPARISON TO VX: Time series data (e.g. Fig. 4) is not always the most accurate representation of how near or far the spacecraft is from the shock. Therefore, we consider another proxy for distance through the transition region, vx (GSE). As plasma encounters the shock, vx increases (absolute vx decreases).
We find that slope decreases in the inertial range when crossing the shock, in the ion range there is no significant change, while the electron range shows a decreasing spectral index. This is the same result as shown by method A & B above.
Fig. 5: Power law index (calculated using method A) against vx for the three ranges. Slope decreases in inertial range, remains mostly unchanged in ion range and decreases in the electron range.
5. CONCLUSION: Across a quasi-parallel bow shock, the magnetic spectrum displays a power law that becomes shallower at the inertial scale, steeper at the electron scale and does not significantly change at ion scales. The electron scale slope steepens to become approximately equal to the ion slope such that the distinction between them is lost.
Authors: James Plank (jp5g16@soton.ac.uk) and Imogen Gingell
1. INTRODUCTION: Reconnection has been observed at Earth’s bow shock, and has also been observed to influence properties of turbulence. At shocks, reconnection can occur in a disordered/turbulent transition region. Observations from Magnetospheric Multiscale (MMS) have been used to identify an event (Fig. 1) that shows in detail how the magnetic spectrum evolves as solar wind plasma crosses a quasi-parallel bow shock and enters the transition region downstream of the shock.
2. POWER LAWS AT DECREASING SCALES: The magnetic spectrum (Fig. 2, 3) displays power law relationships at inertial, ion and electron scales. The steepness of the slope both in relative (compared to smaller/larger scales) and absolute terms is related to turbulent energy cascades.
Fig. 2: Magnetic spectrum in STR (6.7s window). There is a distinct change in power law slope visible at ~ion scale, however, there is no clear change at the electron scale. Slope stays flat from ~ion limit almost down to instrument limit.
Fig. 3: The magnetic spectrum in SW. Multiple power law slopes are visible. Two power laws have been identified by MARS algorithm in the ion range. There is also a separate power law visible for electron scales.
3. EVOLUTION OF SPECTRUM ACROSS SHOCK: By splitting the event into ~6.7s intervals (two examples Fig. 1 & Fig. 2), we can show how power law slopes change for each range of scales.
METHOD A: Assuming spectral breaks occur at k=rho i and k=rho e=de, the slope associated with each region can be calculated with a linear regression (Fig. 4 middle).
METHOD B: Using the MARS algorithm we chain (an arbitrary number of) linear fits end-to-end across the entire spectrum -3.6≤ log k ≤1 (Fig. 4, bottom). The spectral break is not guaranteed to be placed exactly on the ion/electron gyroradius (see Fig. 2 & 3). Hence, this results in a more accurate fit to the magnetic spectrum.
Fig. 4: Middle: Slope for inertial, ion and electron ranges, as calculated by method A. Inertial range slope flattens across the shock. Ion slope does not change significantly. Electron range slope becomes steeper. Bottom: Time dependence of best fit power laws to magnetic spectrum identified by method B (MARS). As plasma enters shock transition region, the distinction between electron and ion ranges is lost.
4. COMPARISON TO VX: Time series data (e.g. Fig. 4) is not always the most accurate representation of how near or far the spacecraft is from the shock. Therefore, we consider another proxy for distance through the transition region, vx (GSE). As plasma encounters the shock, vx increases (absolute vx decreases).
We find that slope decreases in the inertial range when crossing the shock, in the ion range there is no significant change, while the electron range shows a decreasing spectral index. This is the same result as shown by method A & B above.
Fig. 5: Power law index (calculated using method A) against vx for the three ranges. Slope decreases in inertial range, remains mostly unchanged in ion range and decreases in the electron range.
5. CONCLUSION: Across a quasi-parallel bow shock, the magnetic spectrum displays a power law that becomes shallower at the inertial scale, steeper at the electron scale and does not significantly change at ion scales. The electron scale slope steepens to become approximately equal to the ion slope such that the distinction between them is lost.
Poster file