Formation processes of flux ropes downstream from Martian crustal magnetic fields inferred from Grad‐Shafranov reconstruction

Abstract

We applied the Grad-Shafranov reconstruction (GSR) technique to Martian magnetic flux ropes observed downstream from strong crustal magnetic fields in the southern hemisphere. The GSR technique can provide a two-dimensional axial magnetic field map as well as the axial orientation of flux ropes from single-spacecraft data under assumptions that the structure is magnetohydrostatic and time independent. The reconstructed structures, including their orientation, allowed us to evaluate possible formation processes for the flux ropes. We reconstructed 297 magnetic flux ropes observed by Mars Global Surveyor between April 1999 and November 2006. Based on characteristics of their geometrical axial orientation and transverse magnetic field topology, we found that they can be mainly distinguished according to whether draped interplanetary magnetic fields overlaying the crustal magnetic fields are involved or not. Approximately two thirds of the flux ropes can be formed by magnetic reconnection between neighboring crustal magnetic fields attached to the surface. The remaining events seem to require magnetic reconnection between crustal and overlaid draped magnetic fields. The latter scenario should allow planetary ions to be transferred from closed magnetic flux tube to flux tubes connected to interplanetary space, allowing atmospheric ions to escape from Mars. We quantitatively evaluate lower limits on potential ion escape rates from Mars owing to magnetic flux ropes.

MAVEN observations of a giant ionospheric flux rope near Mars resulting from interaction between the crustal and interplanetary draped magnetic fields

“…The inferred flux rope axis was approximately oriented along the + Z mso direction, indicating that it was created via multiple X line reconnection between the local crustal and overlaid interplanetary magnetic fields draped around Mars (Figure ). This formation process was previously proposed by Hara et al []. Hence, these MAVEN observations can convincingly show that this formation process surely operated under the Martian plasma environment. Figure suggested that the giant ionospheric flux rope was detected when an ICME impacted Mars.…”

“…The estimated chirality (handedness) of the flux rope magnetic fields is also consistent with the recovered two‐dimensional structure in Figure c. Although Hara et al [] also pointed out a possibility of this proposed formation process, our results show stronger evidence for this proposed formation process to operate under the Martian ionosphere.…”

“…This definition is the same as Hara et al []. Using the same methodologies as taken by, e.g., Hara et al [, , ], we estimated the spatial scales of the observed ionospheric flux rope. The cross‐section area is computed to be ∼2.06 × 10 4 km 2 .…”

MAVEN observations of magnetic flux ropes with a strong field amplitude in the Martian magnetosheath during the ICME passage on 8 March 2015

“…Hara et al [, ] previously investigated the magnetic flux ropes observed by MGS around Mars to understand the possible ion escape rates associated with flux ropes, although MGS lacked the ion measurements. Recently, Hara et al [] recovered the spatial structure of a detached magnetic flux rope based on the MAVEN simultaneous plasma and magnetic field measurements.…”

On the origins of magnetic flux ropes in near‐Mars magnetotail current sheets

“…Alternatively, we can explore the second possibility that does not necessitate magnetotail reconnection: transport of flux ropes that are originally generated in the dayside ionosphere through plasma instabilities such as helical kink, Kelvin‐Helmholtz instabilities [e.g., Elphic and Russell , ; Wolff et al , ; Ruhunusiri et al , ], and magnetic reconnection in consequence of interactions between the IMF and crustal fields [e.g., Hara et al , , ], or dayside magnetic reconnection analogous to the terrestrial flux transfer events [see, e.g., Hasegawa , , and references therein]. As illustrated in Figure b, these helical draped magnetic field lines can be subsequently transported into the nightside magnetotail forming the current sheet, allowing spacecraft to detect them as flux ropes embedded in the magnetotail current sheets.…”