Mars does not have Earth\u2019s kind of magnetic shield, so it has often been treated as the solar system\u2019s more exposed world. It is a planet left to take the sun\u2019s blows with far less protection. But during a violent solar storm in December 2023, NASA\u2019s MAVEN spacecraft caught Mars doing something scientists did not expect to see so clearly. Instead, it appeared to shove plasma aside inside its own atmosphere.<\/p>\n
The observations offer the first direct evidence that a process called the Zwan-Wolf effect can happen within the ionosphere of an unmagnetized planet. Until now, that effect had largely been associated with places such as Earth\u2019s magnetosphere. This is where a strong internal magnetic field helps redirect the solar wind before it can plunge too deeply toward the planet.<\/p>\n
The new work, led by West Virginia University<\/a> planetary scientist Christopher Fowler, shows that Mars can produce a similar response even without a global dipole magnetic field. Therefore, the finding broadens the picture of how space weather shapes worlds across the solar system, including Venus, Titan and comets.<\/p>\n Christopher Fowler, a WVU planetary scientist, is part of a NASA research team that analyzes data collected from Mars by the twin satellites of the MAVEN mission. (CREDIT: WVU Photo\/Matt Sunday A storm arrives, and Mars responds<\/p>\n The key moment came after an interplanetary coronal mass ejection, or ICME, slammed into Mars on December 9, 2023. That eruption sent a disturbed cloud of solar plasma and magnetic fields racing through space. The Martian environment was left compressed and chaotic hours later when MAVEN sampled it on December 10.<\/p>\n By then, the spacecraft was flying through a magnetosphere still recovering from the impact. The region around Mars was unusually dense, highly variable and magnetically disturbed. Near periapsis, when MAVEN<\/a> dipped to about 185 kilometers above the surface near the terminator, it detected a series of large magnetic structures moving downward through the ionosphere.<\/p>\n Those structures stood out sharply. Their leading edges rose by about 50 nanotesla, roughly 40% of the already strong background magnetic field, then relaxed more gradually on the trailing side. Each sharp leading edge passed the spacecraft in about two seconds. This was followed by a slower recovery lasting roughly 30 to 60 seconds.<\/p>\n At the same moments, MAVEN saw something else. Ion density dropped by about 30% to 40%, while the plasma also shifted direction, flowing downward and tailward. The timing was hard to ignore. The strongest magnetic compression lined up with the local plasma depletion.<\/p>\n That is the pattern Fowler and his colleagues interpret as the Zwan-Wolf effect.<\/p>\n \u201cWhen solar wind, the continuous flow of plasma emitted by the sun, encounters bodies such as planets and comets, it is deflected around them, much like the flow of water in a stream is deflected around a rock,\u201d Fowler said.<\/p>\n But space, he noted, does not behave like water. \u201cBecause the water in that stream is relatively dense, physical collisions between water molecules<\/a> bumping into each other and the rock determine how the water is diverted. In contrast, the environment in space is so tenuous that solar wind particles do not bump into each other. Instead, electromagnetic forces control how particles are deflected around these bodies.\u201d<\/p>\n Overview of the MAVEN orbit analyzed in this study. (CREDIT: Nature Communications) Not above the atmosphere, but inside it<\/p>\n At Earth, the Zwan-Wolf effect happens when incoming magnetic flux tubes are compressed near the magnetopause, creating pressure gradients that squeeze plasma away from the stagnation region. The result is a local thinning of plasma density in front of the planet.<\/p>\n Mars was not expected to show the same thing in quite this way.<\/p>\n Scientists had already reported a depletion region between the Martian bow shock and ionosphere years ago, but the new MAVEN observations go further. They place the action within the ionosphere<\/a> itself, not just in the space above it. The magnetic structures appear to have pushed ionospheric plasma tailward along draped magnetic field lines. This carved out temporary density depletions as they passed.<\/p>\n \u201cThe squeezing helps move the solar wind plasma around the planet, and it makes the plasma less dense in front of the planet,\u201d Fowler said.<\/p>\n \u201cBy finding this effect in the atmosphere of Mars, we are discovering new ways in which our sun can interact with and affect planets in our solar system. It\u2019s amazing to think that an eruption on the sun can disturb the atmosphere of Mars 142 million miles away.\u201d<\/p>\n The team argues that Mars\u2019 induced magnetosphere makes this possible. Even without a global magnetic field, the solar wind drapes magnetic field lines around the dayside of the planet. That draped geometry can produce pressure gradients similar in shape to those near Earth\u2019s magnetopause. During quiet periods, the effect may still be there, just too weak for instruments to pick out. The 2023 solar storm appears to have amplified it enough to become visible.<\/p>\n Schematic summarizing the key processes driving the observed magnetic structures and corresponding plasma depletions. (CREDIT: Nature Communications) Lucky timing, deeper questions<\/p>\n The paper suggests the magnetic structures were likely launched when dynamic pressure pulses in the disturbed magnetosheath struck the magnetic pileup boundary. This action converted pressure changes into magnetic compressions that then propagated downward into the ionosphere.<\/p>\n MAVEN observed the structures all the way to its lowest sampled altitude, implying the effect reached even deeper than the spacecraft could follow.<\/p>\n