Magnetic Effects of Plasma Pressure Gradients in the Upper F Region

Abstract

The Swarm satellites fly at altitudes that at polar latitudes are generally assumed to only contain currents that are aligned with the local magnetic field. Therefore, disturbances along the main field direction are mainly signatures of auroral electrojet currents, with a relatively smooth structure due to the distance from the currents. Here we show that superimposed on this smooth signal is an irregular pattern of small perturbations, which are anticorrelated with the plasma density measured by the Langmuir probe. We show that the perturbations can be remarkably well reproduced by assuming they represent a j × B force, which balances the plasma pressure gradient implied by the density variations. The associated diamagnetic current, previously reported to be most important near the equator, appears to be a ubiquitous phenomenon also at polar latitudes. A spectral analysis indicates that this effect dominates magnetic field intensity variations at small‐scale sizes of a few tens of kilometers.

Plasma patches inside the polar cap and auroral oval: the impact on the spaceborne GPS receiver

“…As already shown in our Figure 3b, the electron temperature within the polar cap patch is reduced by about 500 K, which partly balance the pressure caused by the enhanced plasma gradient, and as a result no large-scale magnetic perturbation is seen within the polar cap patches. While, for the example shown in the Figure 1 of Laundal et al (2019), the electron temperature variation is much smoother than that in our case, therefore, the magnetic residual is well anti-correlation to the plasma gradient. From statistics, they also pointed out that the diamagnetic effect at high latitudes mainly dominates magnetic residual field intensity variations at scale-size of a few tens kilometers.…”

“…However, the 16 Hz electron density data measured by the thermal ion imager of Swarm are not available during the time of interest. A recent paper from Laundal et al (2019) found that the previously reported diamagnetic current at the equatorial region (Lühr et al, 2003;Stolle et al, 2006b) appears to be a ubiquitous phenomenon also at polar latitudes. They showed that the small perturbations of the Swarm magnetic measurements are anti-correlated with the plasma density measured by Langmuir probe.…”

“…where k is the Boltzmann constant, T e and T i are the electron and ion temperatures, n is the electron density and B the magnetic field vector with its magnitude B. By assuming an isotropic pressure and local thermodynamic equilibrium, the magnetic variation dominated by the plasma density gradient can be obtained (Laundal et al, 2019):…”

Long‐Lasting Latitudinal Four‐Peak Structure in the Nighttime Ionosphere Observed by the Swarm Constellation

“…Some of these terms can directly be tested with Swarm, for example, the j × B term, which is often referred to as diamagnetic effect. There are several examples in the literature where the diamagnetic effect provides the balance for plasma structures like the equatorial ionization anomaly (Lühr et al, ), postsunset equatorial plasma bubble (Stolle et al, ), or plasma irregularities at high latitudes (Laundal et al, ). Therefore, we would like to check if such relation is also valid for the four nighttime electron density peaks.…”

Diagnosing low-/mid-latitude ionospheric currents using platform magnetometers: CryoSat-2 and GRACE-FO

“…Also, there are other ionospheric current systems with weaker magnetic effect than the two mentioned above. To name a few, we have solar quiet (Sq) currents flowing in the ionospheric E-layer (Yamazaki and Maute 2017), inter-hemispheric field-aligned currents (IHFACs) connecting the two Sq systems in respective hemispheres (Shinbori et al 2017;Lühr et al 2015Lühr et al , 2019, gravity-driven horizontal currents (Lühr and Maus 2006;Maute and Richmond 2017), pressure-driven currents which counter-balance plasma density inhomogeneity (Lühr et al 2003;Stolle et al 2006;Alken et al 2016;Maute and Richmond 2017;Rodríguez-Zuluaga et al 2019;Laundal et al 2019), wind-driven dynamo currents flowing vertically in the ionospheric F-layer (Lühr and Maus 2006), horizontal currents across the polar cap closing net auroral FACs (Lühr and Zhou 2020, and references therein), and low-/mid-latitude small-scale FACs resulting from a divergence of background currents by ionospheric irregularities (Park et al 2009;Rodríguez-Zuluaga et al 2017;Yin et al 2019). There also exist magneto-hydrodynamic (MHD) waves propagating in the ionosphere and accompanying currents, such as Pc3 (Heilig and Sutcliffe 2016) and Pc1 pulsations (Kim et al 2018;Gou et al 2020).…”