125
These observations allowed Secchi to study the structure of sunspots with
unprecedented detail and accuracy. Indeed, Secchi’s sunspot drawings in Fig. 7.1a
clearly display the penumbral region consisting of bright and dark spines and the
light-bridge over the umbra with striking knot-like dark structures within the central
dark lane. All these features are amazingly reminiscent of the small-scale structural
details reported from high-resolution spectropolarimetric observations of a sunspot
taken by the presently largest solar telescope in operation, the 1.6 m Goode Solar
Telescope at Big Bear Solar Observatory (Zhang et al. 2018, Fig. 7.1c). This demonstrates the excellent quality of Secchi’s observations.
In addition to the complex topology of the sunspot regions, Secchi also described
the granulation pattern observed on the solar surface. He mentioned that this pattern
is disturbed in the areas surrounding sunspots, where faint bright regions are also
seen. These findings are confirmed by current state-of-the-art observations.
Furthermore, Secchi and his assistants regularly recorded data of the area, position,
number and composition of the structures observed over the solar disk (Fig. 7.2).
7.3 The Sun–Earth Connection
Secchi also turned his attention to the dynamics of the observed regions. From his
white light observations performed with the Cauchoix telescope, Secchi recorded
the characteristics of the many structures that appeared on the solar disk from
August 25 to September 6, 1859. He also studied in detail a particular sunspot
region (no. 219 in his register) with a significantly large and complex sunspot group.
Within such a region, Richard Carrington (1826–1875) (Carrington 1859) and
Richard Hodgson (1804–1872) (Hodgson 1859) distinctly witnessed the earliest
white-light flare in observational history on September 1, 1859. This observation
was followed by one of the most extreme magnetic storms experienced by humanity, indicated by a sudden ionospheric disturbance with a consequent increase in
solar energetic particle flux, solar wind velocity, magnetic disturbance, and an auroral display visible on Earth much closer to the equator than normal (Hayakawa et al.
2019). Secchi was among the few contemporary astronomers who monitored the
active region responsible for this extreme space weather storm, today known as the
Carrington event.
His drawings (Fig. 7.3) display his observations of the solar disk and provide
information on the position of the solar equator and poles, on the position and area
of the observed sunspots, faculae, and pores, and various annotations. They record
the observation time, the weather conditions, the estimated heliographic position of
the different observed regions, and some of their characteristics, for example, their
particular brilliance, novel appearance, and fast evolution. In the drawing of his
August 28, 1859 observation (Fig. 7.3a), Secchi recorded the complex nature and
twisted shape of sunspot group no. 219 with small-scale details that he investigated
with a lens inserted into the optics of the telescope to achieve a larger magnification
than that employed for full-disk observations.
7 The Legacy of Angelo Secchi at the Forefront of Solar Physics Research
These observations allowed Secchi to study the structure of sunspots with
unprecedented detail and accuracy. Indeed, Secchi’s sunspot drawings in Fig. 7.1a
clearly display the penumbral region consisting of bright and dark spines and the
light-bridge over the umbra with striking knot-like dark structures within the central
dark lane. All these features are amazingly reminiscent of the small-scale structural
details reported from high-resolution spectropolarimetric observations of a sunspot
taken by the presently largest solar telescope in operation, the 1.6 m Goode Solar
Telescope at Big Bear Solar Observatory (Zhang et al. 2018, Fig. 7.1c). This demonstrates the excellent quality of Secchi’s observations.
In addition to the complex topology of the sunspot regions, Secchi also described
the granulation pattern observed on the solar surface. He mentioned that this pattern
is disturbed in the areas surrounding sunspots, where faint bright regions are also
seen. These findings are confirmed by current state-of-the-art observations.
Furthermore, Secchi and his assistants regularly recorded data of the area, position,
number and composition of the structures observed over the solar disk (Fig. 7.2).
7.3 The Sun–Earth Connection
Secchi also turned his attention to the dynamics of the observed regions. From his
white light observations performed with the Cauchoix telescope, Secchi recorded
the characteristics of the many structures that appeared on the solar disk from
August 25 to September 6, 1859. He also studied in detail a particular sunspot
region (no. 219 in his register) with a significantly large and complex sunspot group.
Within such a region, Richard Carrington (1826–1875) (Carrington 1859) and
Richard Hodgson (1804–1872) (Hodgson 1859) distinctly witnessed the earliest
white-light flare in observational history on September 1, 1859. This observation
was followed by one of the most extreme magnetic storms experienced by humanity, indicated by a sudden ionospheric disturbance with a consequent increase in
solar energetic particle flux, solar wind velocity, magnetic disturbance, and an auroral display visible on Earth much closer to the equator than normal (Hayakawa et al.
2019). Secchi was among the few contemporary astronomers who monitored the
active region responsible for this extreme space weather storm, today known as the
Carrington event.
His drawings (Fig. 7.3) display his observations of the solar disk and provide
information on the position of the solar equator and poles, on the position and area
of the observed sunspots, faculae, and pores, and various annotations. They record
the observation time, the weather conditions, the estimated heliographic position of
the different observed regions, and some of their characteristics, for example, their
particular brilliance, novel appearance, and fast evolution. In the drawing of his
August 28, 1859 observation (Fig. 7.3a), Secchi recorded the complex nature and
twisted shape of sunspot group no. 219 with small-scale details that he investigated
with a lens inserted into the optics of the telescope to achieve a larger magnification
than that employed for full-disk observations.
7 The Legacy of Angelo Secchi at the Forefront of Solar Physics Research