Journal of Geophysical Research: Space Physics
Case and statistical studies on the evolution
of hot ﬂow anomalies
L. L. Zhao
, H. Zhang
, and S. Wang
Institute of Space Physics and Applied Technology, Peking University, Beijing, China,
Geophysical Institute, University
of Alaska Fairbanks, Fairbanks, Alaska, USA,
Space Science Center, University of New Hampshire, Durham, New
Hot ﬂow anomalies (HFAs) frequently observed near Earth’s bow shock are phenomena
resulting from the interaction between interplanetary discontinuities and Earth’s bow shock. We identify
199 HFA events using Cluster data from 2001 to 2010 and divide these events into four categories according
to the dynamic pressure proﬁle, namely, “
,” “M,” and “W” types, where the symbols describe the
proﬁle of the dynamic pressure variations. We present case studies to show the main characteristics of each
type of HFAs. Normalized superposed epoch analyses show that variations of the magnetic ﬁeld magnitude
type HFAs are more dramatic than those of M and W types. The statistical study shows that
the occurrence percentage of mature HFAs in W type HFAs is higher than that of
, and M types.
Superposed epoch analysis result shows that variations of plasma parameters and magnetic ﬁeld of mature
HFAs are more dramatic than those of young HFAs, except for temperature. M and W type HFAs may be the
later evolution stages of
type HFAs; on the other hand, four categories of HFAs may be due to
the fact that the spacecraft crossed an HFA structure along diﬀerent paths.
Kinetic phenomena of hot ﬂow anomalies (HFAs), often observed upstream of Earth’s bow shock, are
characterized by clear ﬂow deﬂection and heated plasmas [Schwartz, 1995; Zong and Zhang, 2011]. HFAs are
regarded as results of the interaction between discontinuities in the solar wind and Earth’s bow shock [Thomas
et al., 1991]. Incident solar wind particles are reﬂected at the bow shock and trapped in the discontinuity.
They interact with the incident solar wind particles and are thermalized, forming a core structure with high
thermal pressure. The hot core region then expands outward and forms a low-density region with low
magnetic ﬁeld magnitude. Such an expansion can form shocks at the HFA boundaries [Paschmann et al., 1988].
Therefore, HFAs have four main characteristics [Facskó et al., 2009]: Comparing with the background solar
wind, (1) the magnetic ﬁeld strength inside the event drops, and there is a visible increase in the magnetic ﬁeld
strength at both boundaries of the event; (2) the plasma velocity decreases and deﬂects from the Sun-Earth
line; (3) the temperature increases dramatically; and (4) the plasma number density also decreases at the
event center and increases at both boundaries, which is similar to variations of the magnetic ﬁeld strength.
For some HFAs, the magnetic ﬁeld strength and the density are only enhanced at the trailing edge [Paschmann
et al., 1988].
It has been recognized that HFAs are universal phenomena, and observations of HFAs at Earth, Saturn [Masters
et al., 2009], Venus [Collinson et al., 2012], and Mercury [Uritsky et al., 2014] have been reported. Uritsky et al.
 showed that the characteristic size of HFAs at diﬀerent planets was controlled by the standoﬀ distance
of the bow shock and/or solar wind conditions.
In recent years, some other types of foreshock phenomena such as foreshock bubbles (FBs) and spontaneous
hot ﬂow anomalies (SHFAs) have been discovered, which can appear very similar to HFAs. FBs form due to
changes in the interplanetary magnetic ﬁeld (IMF) associated with solar wind discontinuities and their interac-
tion with the backstreaming ions in the foreshock prior to these discontinuities encountering the bow shock
[Omidi et al., 2010]. Omidi et al.  presented results from local and global hybrid simulations showing
the formation of the FB. Turner et al.  presented the ﬁrst observations of FBs and compared them with
observations of HFAs. SHFAs diﬀers from classic HFAs in the sense that they are formed in the absence of IMF
discontinuities [Zhang et al., 2013].
• The formation of the four types of
HFAs with diﬀerent structure may be
due to their evolution stages
Q. G. Zong,
Zhao, L. L., Q. G. Zong, H. Zhang,
and S. Wang (2015), Case and
statistical studies on the evolution
of hot ﬂow anomalies, J. Geophys.
Res. Space Physics, 120, 6332–6346,
Received 21 NOV 2014
Accepted 24 JUL 2015
Accepted article online 29 JUL 2015
Published online 19 AUG 2015
©2015. American Geophysical Union.
All Rights Reserved.
ZHAO ET AL. STUDY ON EVOLUTION OF HOT FLOW ANOMALIES