Thomas Woolley
Career Stage
Student (postgraduate)
Poster Abstract
Parker Solar Probe measured ubiquitous magnetic field reversals, known as switchbacks, in the near-Sun solar wind during its first two orbits. These switchbacks appear to occur in patches and could be linked to phenomena near the solar surface, although their precise origin is still unknown. As switchbacks are associated with faster plasma flows, we question whether they follow a typical solar wind temperature-velocity relationship (i.e. are they hotter than the background plasma?). We select time periods with large angular deflections, allowing us to compare the proton parallel temperatures inside and outside switchbacks. We conclude that the proton parallel temperature is very similar inside and outside of switchbacks. This implies that a temperature-velocity relationship does not hold for the proton parallel temperature inside magnetic field switchbacks. We then discuss the possible origins of switchbacks in light of these results.
Plain text summary
Figures Descriptions
Fig. 1 is an artist’s impression of the PSP mission orbiting around the Sun. The distinctive features in the figure are PSP’s heat shield and boom.
Fig. 2 is a five panel (labelled a-e) timeseries plot of a switchback that shows: (a) the angle between the magnetic field vector and the radial, (b) the magnetic field magnitude and the proton core density, (c) the radial components of the magnetic field and the proton core velocity, (d) the normal and tangential components of the magnetic field and (e) the normal and tangential components of the proton core velocity.
The start of the switchback is characterised by a sharp change in the radial magnetic field component (Br) from approximately -75nT to 75nT (panel c). Br then stays around 75nT for approximately 20 minutes before sharply returning to the background field (-75nT). The switchback’s Br profile is similar to a top hat, with steep edges on either side and a relatively stable positive radial magnetic field component in between. The radial velocity profile follows Br very closely which is an indicator that the plasma is Alfvenic (i.e. the velocity and magnetic field fluctuations are highly correlated).
The rotation of the magnetic field is also clear in panel (a) where the angle between the magnetic field vector and the radial direction decreases from around 180 degrees in the background, to approximately 0 degrees inside the switchback. This angle then returns to its background orientation after the switchback.
Throughout the background and switchback plasma, the magnitude of the magnetic field remains somewhat constant, although it exhibits some short lived drops and a slight increase inside the switchback. The proton core density decreases throughout the switchback in order to maintain a similar total pressure to the background plasma.
The behaviour of the tangential and normal components of the velocity and magnetic field (panels d and e) are less important and so not discussed here. It should be noted that they are well correlated with each other as the plasma is Alfvenic.
Fig 3. shows the proton core parallel temperature in the background plasma against the proton core parallel temperature inside the switchback for five different events. Each switchback has two data points associated to it, one with error bars and one without. The data points with error bars are the temperatures obtained using a Gaussian fitting to the SPC-measured distributions. These points, within error, are consistent with the line y=x, indicating that the proton core parallel temperature inside switchbacks is very similar, if not the same, as the background plasma.
The other data points (without error bars) are predictions of the proton core parallel temperature inside of each switchback. These predictions were made by assuming a temperature-velocity relationship for switchbacks, and using the velocity of the switchback and background plasma. These predicted temperatures are much greater than the SPC-measured temperatures, even when considering errors. This suggests that a T-V relationship does not hold within switchbacks for the proton core parallel temperature, and further supports the idea that switchbacks have the same proton core parallel temperature as the background plasma.
From these results we can start to draw conclusions about the origin of switchbacks. For example, this work is more consistent with switchbacks being caused by waves propagating along open field lines than by jets of plasma from different parts of the Sun. This is because plasma from a different part of the Sun would likely have a different temperature to the background, and hence there would be a temperature change inside of switchbacks.
Fig. 1 is an artist’s impression of the PSP mission orbiting around the Sun. The distinctive features in the figure are PSP’s heat shield and boom.
Fig. 2 is a five panel (labelled a-e) timeseries plot of a switchback that shows: (a) the angle between the magnetic field vector and the radial, (b) the magnetic field magnitude and the proton core density, (c) the radial components of the magnetic field and the proton core velocity, (d) the normal and tangential components of the magnetic field and (e) the normal and tangential components of the proton core velocity.
The start of the switchback is characterised by a sharp change in the radial magnetic field component (Br) from approximately -75nT to 75nT (panel c). Br then stays around 75nT for approximately 20 minutes before sharply returning to the background field (-75nT). The switchback’s Br profile is similar to a top hat, with steep edges on either side and a relatively stable positive radial magnetic field component in between. The radial velocity profile follows Br very closely which is an indicator that the plasma is Alfvenic (i.e. the velocity and magnetic field fluctuations are highly correlated).
The rotation of the magnetic field is also clear in panel (a) where the angle between the magnetic field vector and the radial direction decreases from around 180 degrees in the background, to approximately 0 degrees inside the switchback. This angle then returns to its background orientation after the switchback.
Throughout the background and switchback plasma, the magnitude of the magnetic field remains somewhat constant, although it exhibits some short lived drops and a slight increase inside the switchback. The proton core density decreases throughout the switchback in order to maintain a similar total pressure to the background plasma.
The behaviour of the tangential and normal components of the velocity and magnetic field (panels d and e) are less important and so not discussed here. It should be noted that they are well correlated with each other as the plasma is Alfvenic.
Fig 3. shows the proton core parallel temperature in the background plasma against the proton core parallel temperature inside the switchback for five different events. Each switchback has two data points associated to it, one with error bars and one without. The data points with error bars are the temperatures obtained using a Gaussian fitting to the SPC-measured distributions. These points, within error, are consistent with the line y=x, indicating that the proton core parallel temperature inside switchbacks is very similar, if not the same, as the background plasma.
The other data points (without error bars) are predictions of the proton core parallel temperature inside of each switchback. These predictions were made by assuming a temperature-velocity relationship for switchbacks, and using the velocity of the switchback and background plasma. These predicted temperatures are much greater than the SPC-measured temperatures, even when considering errors. This suggests that a T-V relationship does not hold within switchbacks for the proton core parallel temperature, and further supports the idea that switchbacks have the same proton core parallel temperature as the background plasma.
From these results we can start to draw conclusions about the origin of switchbacks. For example, this work is more consistent with switchbacks being caused by waves propagating along open field lines than by jets of plasma from different parts of the Sun. This is because plasma from a different part of the Sun would likely have a different temperature to the background, and hence there would be a temperature change inside of switchbacks.
Poster file
Poster Title
Solar Wind Proton Behaviour inside Magnetic Field Switchbacks
Url
https://www.linkedin.com/in/thomaswoolley1/