ИСТИНА

Primitive asteroids 24 Themis (C-B-type, q = 2.7453 AU), 449 Hamburgа (C-type, q = 2.1118 AU) and 704 Interamnia (F-B-type, q = 2.5828 AU) were accidentally located at heliocentric distances of 2.76 AU, 2.11 AU, and 3.16 AU, in a narrow angular sector (at phase angles of 17÷25°) and near their perihelion distances on 17–19 March 2019. According to our UBVRI-photometry of these asteroids together with a solar analog star (HIP 29759), they had close in shape interpolated reflectance spectra on 17 and 19 March with signs of sublimation activity (two peaks at 0.45 µm and 0.65 µm pointed there to a light-scattering exosphere) (Busarev et al., 2019, 10th Moscow Solar Syst. Sympos., Abs. 10MS3-SW-07). But on March 18 all of the asteroid interpolated spectra turned out to be very other, without signs of activity, close to "canonical" ones defined at SMASS-SMASSII surveys. Worth mentioning, the UBVRI-data were obtained under good photometric conditions. We explained such a strange change in the shape of asteroid spectra on March 18 by the effect of sweeping away of a sublimation exosphere by a shock wave in solar wind (SWSW). It may happen because of two weak solar flares on March 8–9 (according to data of GOES-15 satellite). The first one (March 8, UT = 03h15m) was more powerful and reached Interamnia on March 18 (middle time of the observation UT = 18h37m), as the most distant one, in ~11 days at a speed of ~550 km/s. There were no other solar flares of such intensity prior a month before 8 March 2019 and between 9 and 18 March. Interplanetary magnetic field created by the Sun usually spirals all charged particles moving in the anti-sun direction. But we suggest that when the Earth is between the Sun and asteroids, Earth’s magnetospheric tail, stretched in that direction, should direct the flowing SWSWs nearly in a straight line to asteroids being in opposition. A quasi-neutral charge of solar wind may also be important in this respect. The length of Earth’s magnetotail is still unknown. Based on the above, it can be assumed that it is no less than 2 AU and apparently changes depending on solar activity.