Editing Epsilon Eridani (section)

=== Magnetic activity ===

Epsilon Eridani has a higher level of [[Stellar magnetic field|magnetic activity]] than the Sun, and thus the outer parts of its atmosphere (the [[chromosphere]] and [[solar corona|corona]]) are more dynamic. The average magnetic field strength of Epsilon Eridani across the entire surface is {{val|1.65|0.30|e=-2|u=[[Tesla (unit)|tesla]]}},<ref name=aaa318_429 /> which is more than forty times greater than the {{nowrap|(5–40) × 10<sup>−5</sup> T}} magnetic-field strength in the Sun's photosphere.<ref name=apj591_2_1248 /> The magnetic properties can be modelled by assuming that regions with a [[magnetic flux]] of about 0.14&nbsp;T randomly cover approximately 9% of the photosphere, whereas the remainder of the surface is free of magnetic fields.<ref name=apj1_439_2_939 /> The overall magnetic activity of Epsilon Eridani shows co-existing {{Val|2.95|0.03}} and {{Val|12.7|0.3}} year activity cycles.<ref name=Metcalfe2013 /> Assuming that its radius does not change over these intervals, the long-term variation in activity level appears to produce a temperature variation of 15 K, which corresponds to a variation in [[UBV photometric system|visual magnitude]] (V) of 0.014.<ref name=apj441 />

The magnetic field on the surface of Epsilon Eridani causes variations in the [[hydrodynamic]] behaviour of the photosphere. This results in greater [[jitter]] during [[Doppler spectroscopy|measurements of its radial velocity]]. Variations of {{nowrap|15 m s}}<sup>−1</sup> were measured over a 20&nbsp;year period, which is much higher than the [[measurement uncertainty]] of {{nowrap|3 m s}}<sup>−1</sup>. This makes interpretation of periodicities in the radial velocity of Epsilon Eridani, such as those caused by an orbiting planet, more difficult.<ref name=iau202 />

[[File:EpsEriLightCurve.png|thumb|upright=1.5|left|A [[light curve]] for Epsilon Eridani, showing averages of the [[Photometric system#Photometric letters|b and y band]] magnitudes between 2014 and 2021.<ref name="Roettenbacher"/> The inset shows the periodic variation over a 12.3-day rotational period.<ref name=apj102_5_1813/>]]
Epsilon Eridani is classified as a [[BY Draconis variable]] because it has regions of higher magnetic activity that move into and out of the line of sight as it rotates.<ref name=gcvs /> Measurement of this [[rotational modulation]] suggests that its equatorial region rotates with an average period of 11.2&nbsp;days,<ref name=an328_10 /> which is less than half of the rotation period of the Sun. Observations have shown that Epsilon Eridani varies as much as 0.050 in V magnitude due to [[starspot]]s and other short-term magnetic activity.<ref name=apj102_5_1813 /> [[Photometry (astronomy)|Photometry]] has also shown that the surface of Epsilon Eridani, like the Sun, is undergoing [[differential rotation]] i.e. the rotation period at equator differs from that at high [[latitude]]. The measured periods range from 10.8 to 12.3 days.<ref name=apj441 /><ref name=rotation group=note /> The [[axial tilt]] of Epsilon Eridani toward the line of sight from Earth is highly uncertain: estimates range from 24° to 72°.<ref name=an328_10 />

The high levels of chromospheric activity, strong magnetic field, and relatively fast rotation rate of Epsilon Eridani are characteristic of a young star.<ref name=apj1_412_2_797 /> Most estimates of the age of Epsilon Eridani place it in the range from 200 million to 800 million years.<ref name=aaa488_2_771 /> The low abundance of heavy elements in the chromosphere of Epsilon Eridani usually indicates an older star, because the [[interstellar medium]] (out of which stars form) is steadily enriched by heavier elements produced by older generations of stars.<ref name=aaa358_850 /> This anomaly might be caused by a [[diffusion]] process that has transported some of the heavier elements out of the photosphere and into a region below Epsilon Eridani's [[convection zone]].<ref name=cjaa8_5_591 />

The [[X-ray]] luminosity of Epsilon Eridani is about {{val|2|e=28|u=[[erg]]·s<sup>–1</sup>}} ({{val|2|e=21|u=[[Watt|W]]}}). It is more luminous in X-rays than the Sun at [[Solar cycle|peak activity]]. The source for this strong X-ray emission is Epsilon Eridani's hot corona.<ref name=apj243_234 /><ref name=apj457_882 /> Epsilon Eridani's corona appears larger and hotter than the Sun's, with a temperature of {{val|3.4|e=6|u=K}}, measured from observation of the corona's ultraviolet and X-ray emission.<ref name=mnras385_4_1691 /> It displays a cyclical variation in X-ray emission that is consistent with the magnetic activity cycle.<ref name=Coffaro_et_al_2020/>

The [[stellar wind]] emitted by Epsilon Eridani expands until it collides with the surrounding [[interstellar medium]] of diffuse gas and dust, resulting in a bubble of heated hydrogen gas (an [[astrosphere]], the equivalent of the [[heliosphere]] that surrounds the Sun). The [[absorption spectrum]] from this gas has been measured with the [[Hubble Space Telescope]], allowing the properties of the stellar wind to be estimated.<ref name=mnras385_4_1691 /> Epsilon Eridani's hot corona results in a mass loss rate in Epsilon Eridani's stellar wind that is 30 times higher than the Sun's. This stellar wind generates the astrosphere that spans about {{convert|8000|au|pc|abbr=on}} and contains a [[bow shock]] that lies {{convert|1600|au|pc|abbr=on}} from Epsilon Eridani. At its estimated distance from Earth, this astrosphere spans 42&nbsp;arcminutes, which is wider than the apparent size of the full Moon.<ref name=apj574_1 />