Ion loss to space has played an important role in atmospheric escape and climate change on Mars because of intense solar activity during a younger, more active phase of the Sun. Although the existence of an intrinsic magnetic field on ancient Mars is also a key factor in ion loss, its effect remains unclear. Based on multispecies magnetohydrodynamics (MHD) simulations, we investigated processes and rates of ion loss from Mars under extreme solar conditions and the existence of a dipole field with different strengths. The effects of a dipole field on ion loss depend on whether the dipolar magnetic pressure is strong enough to sustain the solar wind dynamic pressure. When the dipole field is existent but weak, it facilitates the cusp outflow and increases the loss rates of molecular ions (O2+ and CO2+) by a factor of 6 through the high‐latitude magnetotail. When the dipole field is strong enough, the loss rates of molecular ions are decreased by 2 orders of magnitude, and peaks of the escape flux are located near the equatorial plane due to the magnetic reconnection in the northern‐dusk or southern‐dawn lobe regions. The pickup process on the extended oxygen corona created by the strong EUV flux contributes to the total O+ loss. Therefore, the effects of the dipole field are less pronounced for O+. Under more moderate solar EUV conditions, the effects on O+ loss can be stronger and thus contribute to climate change.

The planetary intrinsic magnetic field is critical when considering the atmospheric escape from planets. The strength of the intrinsic magnetic field particularly affects the interaction between solar wind and terrestrial-type planets (e.g., Chassefière & Leblanc, 2004; Seki et al., 2001), and it changes the escape mechanism. The terrestrial global magnetic field has also experienced strength changes (e.g., Guyodo & Valet, 1999) and recurring reversals over 4.6 billion years (Gyr) that could have affected the atmospheric composition of planets. It is believed that ancient Mars had a global intrinsic magnetic field of interior origin and the magnetic field decayed by ∼3.9 Gyr ago (Acuña et al., 1999). One of the pieces of evidence that ancient Mars had an intrinsic field is the existence of a "crustal magnetic field" (Acuña et al., 1999). Present-day Mars has a remanent magnetism in the crust mainly in the southern hemisphere, which is called the crustal magnetic field. The relationship between planetary climate change and the existence of intrinsic magnetic fields is an interesting research topic. It is considered that Mars had maintained a warm and wet climate until ∼4 Gyr ago (e.g., Goldspiel & Squyres, 1991; Jakosky et al., 1994; Malin & Edgett, 2003). However, the atmosphere and water were lost, resulting in only a thin Martian atmosphere. A recent model study suggests that Mars lost a large portion of the atmosphere within 500 million years of its origin (Lammer et al., 2013). The atmospheric escape can be separated into the neutral and ion escape. The neutral escape channels include the Jeans escape and hydrodynamical escape associated with the escape of relatively light species such as hydrogen,

It is thought that Mars had a warm and aqueous environment and experienced a drastic climate change during its early period (Kite, 2019;Wordsworth, 2016). Ancient Mars was exposed to intense solar conditions such as the solar X-ray and extreme ultraviolet (XUV) radiation (Ribas et al., 2005;Tu et al., 2015) and the mass loss rates (Wood, 2006). Intense solar conditions during the early period of the solar system should have enhanced heating and ionization of the planetary upper atmosphere and thus atmospheric escape to space, particularly escape of the ionized atmosphere, that is, ion escape (Jakosky et al., 2018;Lammer et al., 2013). Terada, Kulikov et al. (2009) simulated ion escape on Mars under extremely strong solar conditions at 4.5 Ga and estimated that the ion escape rate reaches 10 28 -10 29 s −1 , several orders of magnitude higher than the present rate at Mars (10 24 -10 25 s −1 ) (Brain et al., 2015;Nilsson et al., 2011;Ramstad et al., 2015). In addition, solar events such as coronal mass ejection (CME) events occurred more frequently at young Sun (Kay et al., 2019). The observational and numerical studies

We investigated the formation mechanism of a molecular ion plume and its contribution to ion escape based on Mars Atmosphere and Volatile EvolutioN (MAVEN) observations from November 2014 to October 2019 and numerical models. Here, we report a CO2+‐rich plume event and a statistical study of the molecular ion plume. MAVEN observed a CO2+‐rich plume event, in which the maximum CO2+ escape flux is approximately 4.2 × 106 cm−2s−1, on 28 August 2015 under strong solar wind dynamic pressure conditions. A numerical simulation using strong solar wind dynamic pressure conditions from the event suggested that the molecular ion plume is formed by deep penetration of the solar wind‐induced electric field, which is caused by strong solar wind dynamic pressure. A statistical study showed that CO2+ plume events tend to be observed under high solar wind dynamic pressure and strong electric field conditions. This tendency is consistent with the formation mechanism of the molecular ion plume suggested by the event study. The O2+ plume does not show the same tendency. This is because O2+ ions are abundant in the high‐altitude ionosphere, and O2+ plumes can be formed even under weak solar wind conditions. The subsolar crustal magnetic fields tend to prevent the formation of the molecular ion plume by shielding the ionosphere from the solar wind. The escape rate ratio ()normalO+:normalO2+:CO2+ $\left({\mathrm{O}}^{+}:{\mathrm{O}}_{2}^{+}:{\text{CO}}_{2}^{+}\right)$ is approximately 45:53:3 during the whole statistical survey period, suggesting that a molecular ion plume from the ionosphere is a non negligible ion escape channel from Mars.

Mars, a weakly magnetized planet, is affected by interplanetary magnetic field (IMF) rotation in terms of its interaction with the solar wind. Observations by Mars Atmosphere and Volatile EvolutioN (MAVEN) indicate that its magnetotail lobes are twisted ∼45° from the ecliptic plane up to a few Mars radii downstream due to the IMF orientation (DiBraccio et al., 2018(DiBraccio et al., , 2022. Based on hybrid simulations, the bow shock is adopted instantaneously to the new solar wind conditions, while the magnetic pileup boundary and the magnetic lobes require up to 1-2 min (Modolo et al., 2012). Romanelli et al. (2018Romanelli et al. ( , 2019 suggested from a comparison between MAVEN observations and hybrid simulations that recovery timescales for an ∼90° IMF rotation that lasted 50 s are between 8 s and 11 min, depending on the magnetospheric region considered, while planetary H + and O + loss rates do not show strong changes in IMF orientation variations.Strongly magnetized planets such as Earth are particularly susceptible to IMF orientation because the global intrinsic magnetic field interacts with the IMF. Ogino et al. (1994) showed through a global magnetohydrodynamic (MHD) simulation that the magnetic reconnection site moves from the subsolar point to the high-latitude tail due to the IMF rotation from south to north on Earth. The transport of the reconnection site affects the ion escape mechanism. Turning the IMF from south to north also generates a cold-dense plasma sheet, in which the density of the near Earth plasma sheet typically increases by a factor of 3 and the ion temperature decreases from a few keV to 1 keV or less (e.g., Øieroset et al., 2005; Terasawa et al., 1997), and the theta aurora made up of transpolar arcs and the oval (e.g., Tanaka et al., 2004) on Earth. On the other hand, Sakai et al. ( 2021) found under weak intrinsic magnetic field conditions that the IMF orientation is critical for ion escape from a Mars-like planet. The northward (parallel to the dipole at the surface) IMF quenches escape from the upper atmosphere and the escape rate is over an order of magnitude smaller in the typical Parker-spiral or southward (antiparallel) IMF cases because the Earth-like magnetosphere is formed and protects the atmosphere in the northward IMF