Indian Journal of Radio & Space Physics Vol. 28, February 1999, pp . .36-42 Field aligned current and parallel electric field between magnetosphere and ionosphere of Mars S A Haider, S P Seth* & K S Raina Physical Research Laboratory, Ahmedabad 380 009 Recei ved 2 September 1998; revised received II December 1998 A kineti c model is used to estimate the electron current density at different potential differences between the and ionosphere of Mars. For this calculation it is assumed th at Mars has a we ak dipole magnetosphere whose plas ma sheet is connected to the ionosphere along the magnetic field lines. The total electron flux of cm- 2 S- I cal cu lated in the Marti an magnetosphere corresponding to the downward current density of 1.0x I0 -{i A/m 2 at zero potential dillercnce. The maximum flux of thermal electrons escaping towards th e plasma sheet of Mars at zero current de ns it y is c il c ul ated to he 3.0x I OX cm- 2 S- I. At zero current density the precipitati ng flux of plasma sheet electrons is approxilllately equal to max imum escape flux of thermal electrons. The calculated r es ults are found to be in reasonable agreel11cnt with the experimcntal data obtained from Hyperboli c Ret arding Potential Analyzer (HARP) expe rime nt onboard PIHlhos-2 . I Introduction The solar wind inte ra c ti on with Mars is hi ghly co nt rove rs ial mainl y beca us e of insufficie nt data. The measure lll c ni s fr om Mars-2,-3 and -5 have been used to prov id e evidence that Mars ha s an intrinsic mag net ic fi e ld wit h a d ipole mome nt 1. 1 in th e range of 1.:iO-2 .2x IOI :igauss em' and that th e field can be effective obstacle to th e so lar wind . Rela ti ve ly large obs tac le height and lack of ionopause-like signatures in th e pla\ ma de. ns ity 1 have also been considered to suppo rt thi s argument. Recent results from Phobos-2 encoutller indicate that Mars should have hybr id whic h is the co mbinati on of intrin sic ma gnet ic field emanating from th e pole of a weak dipo le a nd Ve nu s- like draped interplanetary fi e ld S suppl y ng in th e ne ig hb ourhood of th e ma gnetic eq uat or. The case for direct inte ra c ti on of so lar wind wi th Mars ha s been di scussed by Luhmann et al .f> Th ey pointed o ut that Mars' ionos ph ere is weak and solar wind pressure exceeds th e ionosphe ri c pressure most of the time. The situat io n is thus sim il ar to solar maximum of Venus ionos ph ere under hi gh solar wind pressure. Shinagawa and Cravens 7 have pro po sed an one-dimensional magneto-hydrodynamic model of th e Martian ionosphere and concluded that Mars" *Pe rillall cllll y at Bhavan' s R A Co lk ge of Science, Ahmedabad .1ilO OOl) intrin s ic magnetic fi e ld is very small which does not playa major ro le in solar wi nd interaction . On the other hand Lundin et {(I . R, I) and Barabash et al. 10 have found striking similarities between ionospheric outflows from Mar'S and Earth. Lammer and Bauer l t use d an intrinsic magnetos ph ere for the transportation of ionospher ic ions fr om th e days ide cusp region to th e plasma sheet of Mars. [n th e present study also an intrins ic magnet ic fi e ld has been used to calculate polar wind flux and dens iti es I2 . 13 at Mars. This calculati on suggests that physical process of ion s escap in g from th e ionosphere of Mars and Earth at hi gh latitude should be the same insp it e of sma ll Martian ma gnetosphere compared to that of Ea rth . Th e magnetosphe ri c plasma is essentiall y collision less. In such a plasma, the parallel elect ri c fi e ld fr eely accelerate particles along the magnetic field lin es between th e ionosphere and pla'sma sheet. The electrons and ions are accelerated in opposit e directions givin g rise to a current along the n1a gnetic fi e ld lines. The e lect ron s make th e dominant contribution to net electric curre nt because th e mobility of th e ion is lower than the mobility of th e e lectro ns. In this paper, a kinetic model is used to calculate the parallel e lectric current carried by electro ns along th e int ri nsic magne ti c fi e ld lines connecting- th e ni ghttime ionosphere of Mars with th e plasmashee t. Th e va lidit y of this model res ts upon th e assumption that the scale of ma gne ti c fi e ld gradients