Is there a difference in Earth's magnetic field between day and night? Is there a difference in Earth's magnetic field between day and night? 
Magnetic Field
 A: It looks like there is some difference between day and night geomagnetic field, and one of the causes is mentioned in @JEB's comment.
Yamazaki Earth, Planets and Space (2022) 74:99

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)."

A: 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.
