The nearest images of the Sun obtained to date have revealed a new phenomenon, dubbed campfires, which appear as small, bright flickers of light all over the Sun's surface. The Solar Orbiter, a European Space Agency spacecraft launched in February 2020, captured the images from a distance of 77 million km (48 million mi), or about half the distance between Earth and the Sun. The Solar Orbiter will gather vastly more and better observations of campfires as the probe's mission continues for at least the next seven years, with a flight path that will ultimately take the probe to a distance of less than 0.3 astronomical units from the Sun, reaching inside the orbit of Mercury. [An astronomical unit is the average Sun–Earth distance of about 150 million km (93 million mi)]. See also: Astronomical unit
Campfires look like miniature versions of solar flares—large, bright regions easily seen on the Sun from Earth via solar observatories. Flares are caused by magnetic fields accelerating particles including electrons, protons, and heavier nuclei. Researchers do not yet know, however, if the same mechanism is behind both phenomena. While flares can send out bursts of energetic particles that are sufficiently intense to damage satellites in Earth's orbit and trigger massive auroras, the energy output of campfires is not yet quantified. Initial speculation, though, is that because campfires are ubiquitous on the Sun's surface, their combined output could be tremendous, and possibly of high enough intensity to explain an enduring mystery surrounding the solar corona, the outermost portion of the Sun’s atmosphere. The corona’s temperature is about 1 million °C (1.8 million °F), making it significantly hotter than the Sun's surface, which is only about 5700°C (10,000°F). An energy source for the corona has long eluded explanation, but campfires could prove to be the critical missing link. See also: Aurora; Satellite (spacecraft); Solar corona; Temperature