# Adjustments to SIF Aid the Interpretation of Drought Responses at the Caatinga of Northeast Brazil
## 1 Introduction  
Monitoring and measuring vegetation responses to climate has long been an area of interest for biologists and ecologists and it merits attention considering the ongoing global climate change and the closely related interplay between carbon flux, climate and the biosphere [1]. Among the plethora of methods that can be used to assess vegetation responses to climate, remote sensing offers great advantage over field techniques in regard to the size of the population that can be sampled in a given time interval since it allows the sampling of large areas, including remote places and also allows for frequent revisits that would be impractical for field campaigns considering logistics and costs. Since photosynthesis is the main biological process powering the global carbon cycle, the study of vegetation photosynthesis and productivity is paramount for the understanding of carbon fluxes [1]. The remote estimation of chlorophyll \(a\) fluorescence shows promise as a key parameter for sampling global photosynthesis [2; 3]. Chlorophyll fluorescence is an electromagnetic emission that results from the conversion of Sun light energy into electrochemical energy by the plant's chlorophyll molecules. This conversion however is not complete and, while part of the received solar energy is channeled into plant metabolism, part is lost as heat and another part is re-emitted as photons of lower energy and, consequently, longer wavelength [4]. That emission is chlorophyll fluorescence. The energy yielded from that conversion process must be dissipated or used in the plant's metabolism or else it results in the formation of damaging super-oxide compounds within plant cells [5]. Therefore, plant biochemistry imposes negative feedbacks on the energy conversion process that acts at different temporal scales to allow it to adapt to the changing environmental conditions of the daily and seasonal cycles of plant life. In this manner, energy conversion rate for photosynthesis is kept proportional to the viable photosynthetic rate within the given environmental conditions and plant biochemical limitations [6]. Consequently, chlorophyll fluorescence emission is closely related to photosynthetic yield and productivity [2; 7; 8]. Fluorescence has been successfully used to study photosynthesis responses to: environmental stress, herbicides, diseases and pollution [7; 8]. For about nine decades since [PERSON] and [PERSON] published their seminal work describing the phenomena, chlorophyll fluorescence has been studied in laboratories and on the field, at the leaf level, plant level and canopy level with a variety of instruments [9; 10; 11]. With the advancement of remote sensing technology, it has recently become possible to measure this fluorescent emission remotely, from instruments that can be installed in towers, mounted on aircrafts and on satellites [2]. In fact, many satellite-mounted instruments not originally designed for measuring chlorophyll fluorescence are now being used for this end with varying degrees of success, including: GOME, GOME-2, GOSAT, OCO-2, SCIAMACHY, TanSat (CarbonSat) and TROPOMI. Furthermore, the European Space Agency is currently developing the first orbital mission exclusively dedicated to measuring chlorophyll fluorescence from space: FLEX, the FLUorescence EXplorer mission. While leaf-level fluorescence is reasonably well understood [8], the interpretation of remotely sensed emissions presents new challenges and limitations [11; 12]. Firstly, while plant biologists can control energy input to leaves being measured (i.e., PAR level) with lab or field-grade fluorometers [8], the remote sensing of chlorophyll fluorescence depends on Sun light and its natural variability. Therefore, the so-called Sun-Induced Fluorescence (SIF)--equivalent to fluorometer-measured steady-state fluorescence--is what can be remotely estimated by satellite- and aircraft-mounted instruments [2].