Is there a difference in Earth's magnetic field between day and night?
It looks like there is some difference between day and night geomagnetic field, and one of the causes is mentioned in @JEB's comment.
The Earth’s upper atmosphere is weakly ionized, as it receives energy inputs from the Sun in the form of electromagnetic waves. Ionized particles interact with neutrals by collisions and move through the ambient geomagnetic field, which gives rise to an electromotive force to support electric fields and currents. The process is known as the ionospheric wind dynamo, or simply ionospheric dynamo, and it is the dominant production mechanism of ionospheric electric fields and currents at middle and low latitudes during geomagnetically quiet periods (e.g., Richmond 1995a; Heelis 2004). The dynamo currents flow mainly on the dayside at E-region altitudes (ca 90–150 km), where the electrical conductivity of the ionosphere is greatest. At night, the ionospheric conductivity is smaller by about two orders of magnitude (e.g., Richmond 2011). Thus, the currents are also much weaker and have a negligible efect on the geomagnetic feld on the ground. The daytime presence and nighttime absence of the magnetic effect associated with ionospheric dynamo currents lead to daily variation of the geomagnetic field measured at ground stations. Geomagnetic daily variation is smooth and regular in appearance on geomagnetically quiet days when high-frequency geomagnetic disturbances associated with geomagnetic storms and substorms are absent, and is often referred to as solar-quiet (Sq) variation (e.g., Campbell 1989; Yamazaki and Maute 2017)."
Solar wind is constantly out-flowing from the sun. That wind distorts the Earth's magnetosphere by compressing it on the windward (day) side and elongating it on the downwind (night) side. As a result, the shape of the magnetosphere looks like a teardrop when observed from space at a position above the terminator.
During times when the solar wind increases, like when there is a solar flare, that elongation becomes more exaggerated until it can no longer sustain itself. At that point, just like a slingshot that has been pulled back and released, the magnetic field lines compress on the downwind (night) side forcing ionized particles into the atmosphere resulting in an aurora borealis. In the opposite direction, a jet of plasma is released into space in the downwind direction. This is called "plasma farting" by the scientific illuminati.
Ok, I made that last sentence up. But, the rest of it is accurate.